In considering the mode of distribution of organic beings over the surface of the earth, the first important fact which strikes us is that neither the resemblance nor the dissimilarity of the inhabitants of different regions from climatic a.o. physical conditions can be explained. All who have studied this subject have at last reached the same conclusion. The example of America alone would suffice to prove this. For all authors are agreed that, excluding the northern part, which is fairly contiguous around the pole, the separation of the old and new worlds forms one of the first bases of the geographical distribution of organisms. But if we take the wideWandering across the American continent from the central parts of the United States to its southernmost point, we encounter the most varied living conditions, the wettest stretches and the driest deserts, high mountains and grassy plains, forests and marshes, lakes and streams almost any temperature. There is scarcely a climate or condition in the old world which is not paralleled in the new , at least so similar as is required for the progress of the same species; for it is an extremely rare case to find any group of organisms confined to a small spot with somewhat peculiar conditions of life. So e.g. B. There is in the old worldsome places, hotter than any in the new ; and yet these have no peculiar character[p. 379]fauna or flora. But notwithstanding this parallelism in the living conditions of the old and new worlds, how different are their living inhabitants!
If in the southern hemisphere we compare large tracts of land in Australia , South Africa , and western South America between 25°-35° S. latitude, we shall find some parts extremely similar in all their natural proportions, and yet it would not be possible to find three dissimilar faunas and floras. Or if we compare the natural products of South America in the south of 35° and in the north of 25°, which therefore inhabit very different climates, they show themselves to be far more closely related than those in Australia and Africaare living in almost the same climate. And analogous facts can also be demonstrated in relation to the sea creatures.
The second general fact which strikes us is that barriers of various kinds, or hindrances to free migration, are closely and essentially related to the differences between the populations of different districts. Such is the great difference between almost all the land-dwellers of the old and the new world, with the exception of the northern parts, where both almost touch one another and where the migrations of the inhabitants of the north-temperate zone might previously have been possible in a similar way with a climate that differed only slightly , as they are still taking place now on the part of the Arctic population. We recognize the same fact in the great diversity between the inhabitants of Australia , Africa , andSouth America again; for these regions are almost as completely separated from each other as is ever possible. Also on every continent we see the same phenomenon; for on the opposite sides of high and connected mountain ranges, great deserts, and sometimes even just great rivers, we find different products. Since, however, mountain ranges, deserts, etc., are not quite impassable or not as long as those between the[p. 380]If there are oceans on continents, these differences are much smaller in degree than those on different continents.
If we turn to the sea, we find the same law. No other two marine faunas are so different from each other as those on the eastern and western coasts of South and Central America . There is almost no fish, no snail, no crab in common. And yet these great faunas are separated only by the narrow Isthmus of Panama . To the west of the American shores stretches a wide and open ocean, with not an island as a resting place for emigrants; here we have a barrier of a different kind, and as soon as this is crossed we find ourselves on the eastern islands of the Pacific to a new and very different fauna. Thus three marine faunas extend not far from each other in parallel lines far north and south in corresponding climates. But as they are separated from one another by impassable barriers of land or sea, they remain entirely distinct from one another. But if we go further west from the eastern islands in the tropical part of the Pacific , we find no more impassable barriers; innumerable islands or coherent coasts offer themselves as resting places, until we have migrated around a hemisphere to the coasts of Africareach; but no well-characterized distinct marine faunas are divided into these vast areas. Though scarcely a snail, crab, or fish is common to those three faunas on the east and west coasts of America and in the eastern part of the Pacific , yet many species of fish range from the Pacific to the Indian Ocean , and are many mollusks common to the eastern islands of the South Seas and the eastern coasts of Africa under almost exactly opposite meridians.
A third great fact, already partly implied in the foregoing, is the affinity between the products of the same continent or ocean, although the[p. 381]Types of different parts and locations of the same are different. It is a law of the greatest generality, and every continent offers innumerable evidence of it. Notwithstanding this, the naturalist, on his way from north to south, is inevitably struck by the manner in which groups of organisms, distinct in species but apparently allied, successively replace one another. He hears similar songs from closely related but different birds, sees their similarly built nests with similarly colored eggs. The plains of the Strait of Magellan are inhabited by a nandu (Rhea Americana), and north of the plain of Laplata inhabits another species of the same clan, but not a true ostrich (Struthio) or emu (Dromaius), which inAfrica and New Holland , respectively, occur at the same latitudes. In the same Laplata plains we find the agouti (Dasyprocta) and the hare mouse (Lagostomus), two rodents of the habits of our hares and rabbits, and belonging to the same order with them, but forming a purely American organizational type. If we climb up to the high mountains of the Cordillera , we meet the mountain hare mouse (Lagidium); looking about the water, we find two other South American types, the coypu (Myopotamus) and capybara (Hydrochoerus), instead of the beaver and muskrat. Countless other examples could be cited. However much the islands at the American coasts may differ in their geological structure, their inhabitants are essentially American , though of peculiar species. If we look back to the next earlier time periods, as discussed in the last chapter, we still find American types predominant on the American mainland as well as in American seas. We recognize in these facts a deep-seated organic bond, predominant in time and space over given planes of land and water, whatever their natural condition. The naturalist need not be very inquisitive if he does not feel tempted to investigate this volume more closely.
According to my theory, this bond consists only in the[p. 382]Heredity, that cause which alone, as far as we know with certainty, produces organisms like or like the varieties. The dissimilarity of the inhabitants of different districts will be due to the transformation by natural selection, and in a quite subordinate degree to the direct influence of external conditions of life. The degree of dissimilarity depends on whether the migration of the more dominant forms of life from one region to another was more rapid or slower in later or earlier times; it depends on the nature and number of earlier immigrants, on their effect and repercussion in the mutual struggle for existence, since, as I have often remarked, the relation of organism to organism is the most important of all relations. In the migrations, the barriers mentioned above come into play, as does time in the slow process of natural breeding. Widespread and abundant species, which have already triumphed over many competitors in their own extensive homelands, will have the best chance of gaining new places on advancing into new districts. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas. like time in the slow process of natural breeding. Widespread and abundant species, which have already triumphed over many competitors in their own extensive homelands, will have the best chance of gaining new places on advancing into new districts. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas. like time in the slow process of natural breeding. Widespread and abundant species, which have already triumphed over many competitors in their own extensive homelands, will have the best chance of gaining new places on advancing into new districts. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas. who have already triumphed over many competitors in their own extensive homelands, will have the best prospect of gaining new places in advancing into new districts. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas. who have already triumphed over many competitors in their own extensive homelands, will have the best prospect of gaining new places in advancing into new districts. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas. Under the new conditions of life of their future home, they will frequently experience new modifications and improvements; they will become even more superior to the others and produce groups of variant offspring. From this principle of progressive inheritance with modification it follows how it happens that sub-kinships, tribes and even whole families appear, as is so customary and recognized as the case, to be limited to certain areas.
As remarked in the last chapter, I believe in no law of necessary perfection; Just as the variability of species is an independent property and is only exploited by natural selection to the advantage of individuals in their many-sided struggle for existence, so there is no equal measure for the modification of different species. if e.g. B. a number of species in direct competition with each other en masse for a new and afterwards isolated country[p. 383]emigrate, they will experience little modification, since neither migration nor isolation in themselves do anything. Those principles come mainly into consideration when organisms are brought into new relationships with one another, less so when they are brought into contact with new living conditions. As we saw in the last chapter that some forms have maintained their character almost unchanged since immensely remote geological periods, so too some species have migrated over large spaces without undergoing great changes.
From these views it is evident that different species of a family, though inhabiting the remotest parts of the world, must originally have sprung from the same source, having originated from the same progenitor. As to these species, which have changed but little during whole geological periods, there is no difficulty in supposing that they have migrated from the same country; for during the great geographical and climatic changes which have been going on since time immemorial, migrations of any distance have been possible. But in many other cases, where we have reason to believe that the species of a genea arose only in comparatively recent times, the difficulty is far greater. Likewise, it is evident that individuals of a species though they now dwell in far apart and isolated regions, must have emanated from a place where their elders first arose; for, as explained in the last section, it is unbelievable that specifically equal individuals can descend from different ancestral species.
So we have arrived at the question, which is often discussed recently by natural scientists, whether species have ever been produced at one or more places on the earth’s surface. No doubt there may be very many cases where it is extremely difficult to conceive how the same species could have migrated from one point to the various distant and isolated districts where it is now found. notwithstanding[p. 384]the idea that each species must have proceeded from only one original place of birth imposes itself on the mind through its simplicity. And whoever rejects it, rejects the vera causa, ordinary procreation followed by wandering, in order to take refuge in a miracle. It is generally admitted that the area inhabited by a species is usually continuous; and when a species of plant or animal inhabits two points so remote, or separated by such barriers, that it cannot easily have migrated from one to the other, this is regarded as something remarkable and exceptional. The ability to migrate over sea is perhaps more limited in land-mammals than in any other organic being; and consequently we find no inexplicable case where the same species of mammals inhabited very distant points of the world. No geologist finds any difficulty in assuming thatGreat Britain was formerly connected with the European continent, and consequently possessed the same mammals. But if the same species can be produced at two distant points in the world, why not find a single species of mammal common to Europe and Australia or South America ? The conditions of life are nearly the same, so that a multitude of European plants and animals live in America and Australiahave been naturalized, and even some of the native plant species are exactly the same at these two so remote points in the northern and southern hemispheres! The answer lies, I believe, in mammals being unable to make the migration, while some plants, with their various means of dispersal, have been able to traverse this wide and broken interval. The powerful influence exerted by geographical barriers of all kinds on the mode of distribution can only be understood on the supposition that by far the greater part of the species was produced on one side only and had no means of migrating to the other side. A few families, many[p. 385]Subfamilies, very many genera, and a still greater number of subphyla, are each confined to a single locality, and it has been remarked by several naturalists that most natural genera, namely, those whose species are all most closely allied, are only local or at least tend to be dependent on a contiguous area. Now what a wondrous anomaly it would be if, down a stage down the line, the individuals of a species behaved in almost opposite ways, and the species had not been produced locally, but in two or more quite different localities!
Hence it seems to me, like so many other naturalists, that the more probable view is that each species originated in a single country, but has afterwards migrated as far as means and subsistence under past and present conditions will permit. There are undoubtedly still many cases where it is impossible to explain how this or that species got from one place to another. But geographic and climatic changes, which are reliably occurring in recent geological times, must have broken the earlier continuity of the ranges of many species. Thus we come to consider whether these exceptions to the continuity of ranges are so numerous and so important that we are compelled to give up the opinion made probable by the foregoing considerations, that each species originated in only one field and from there migrated as far as possible? It would be desperately tedious to enumerate and discuss all the exceptional cases in which one and the same species now lives in various distant places; nor do I want to claim for a moment that I can really give a sufficient explanation for many of these cases. But, after some preliminary remarks, I would like to discuss the most important classes of such facts, such as, in particular, the occurrence of one species on the crests of distant mountain ranges, be forced to give up? It would be desperately tedious to enumerate and discuss all the exceptional cases in which one and the same species now lives in various distant places; nor do I want to claim for a moment that I can really give a sufficient explanation for many of these cases. But, after some preliminary remarks, I would like to discuss the most important classes of such facts, such as, in particular, the occurrence of one species on the crests of distant mountain ranges, be forced to give up? It would be desperately tedious to enumerate and discuss all the exceptional cases in which one and the same species now lives in various distant places; nor do I want to claim for a moment that I can really give a sufficient explanation for many of these cases. But, after some preliminary remarks, I would like to discuss the most important classes of such facts, such as, in particular, the occurrence of one species on the crests of distant mountain ranges,[p. 386]or in the Arctic and Antarctic circles at the same time; then, secondly (in the following chapter), the wide distribution of fresh-water dwellers, and thirdly, the occurrence of the same terrestrial species on continents and islands separated by hundreds of miles of open sea. If the occurrence of one and the same species at remote and isolated sites on the surface of the earth can in many cases be explained by supposing that the species migrated thither from its place of birth, then, considering our utter ignorance, seems to me to agree the earlier geographical and climatic changes, as well as with some fortuitous means of transport, the assumption that this was the general rule to be by far the most correct.
In discussing this subject we shall have occasion to consider another point of equal importance to us, viz. whether the several different species of a genea which, according to my theory, had a common progenitor, proceeded from (and by way of) the abode of their progenitor may have further developed appropriately). If it can be shown that a region, most of whose inhabitants are closely related or from the same clan with the species of a second region, probably once received immigrants from this latter region, then Diess will contribute to the confirmation of my theory; for we then understand clearly from the modification principle why the inhabitants of one region are related to those of the other, since they come from there. A volcanic island B. which rose a few hundred miles from a continent, would probably, in the course of time, receive some colonists, whose descendants, though somewhat modified, would pass on to their descendants their kinship with the inhabitants of the continent. Cases of this kind are common, and, as we shall see hereafter, inexplicable on the theory of independent creation. This view of the relationship of the species of a region to those of a inexplicable according to the theory of independent creation. This view of the relationship of the species of a region to those of a inexplicable according to the theory of independent creation. This view of the relationship of the species of a region to those of a[p. 387]otherwise (if we now use the word variety instead of species) is not very different from that put forward by Mr. W ALLACE , according to which “each species arose in time and space concurrently with an formerly existing closely allied species.” I now know from his correspondence that he attributes this “concurrence” to the generation with modification, and concedes a long geological time period for it.
The foregoing remarks on single or plural centers of creation do not lead immediately to another related question, namely, whether all individuals of a species descend from a single point or hermaphrodite, or whether, as some authors suppose, from many individuals co-arisen a kind? In those organisms which never interbreed (if such exist at all), the species must, by my theory, arise from a succession of perfected varieties, which have never interbred with other individuals or varieties, but have simply supplanted one another, so that on each of the successive Modification and Improvement Levels all individuals of the same variety must also come from the same progenitor. In the majority of cases, however, and particularly in all organisms which mate for each single reproduction, or interbreed frequently with others, I believe that during the slow process of modification the individuals of the species at interbreeding have remained nearly uniform, so that many of these varied at the same time, and the whole amount of variation at each stage did not result from descent from a common ancestor. To illustrate what I mean, I will say that our English thoroughbred horses differ but little from the horses of every other breed, but derive their difference and perfection not from their descent from a single pair, but the same from that of many Generations of care taken in selecting and raising many individuals.
Before proceeding to a more detailed discussion of those three classes of facts constituting the theory of the “single[p. 388] Centers of Creation” present the most difficulties, I must say a few words about the means of dissemination.
Diffusion Agent. ) Sir C H . L YELLetc. Authors have discussed this subject very appropriately. I can only give a brief excerpt of the most important facts here. Climate change may have had the greatest influence on the migration of organisms. A region of changing climate may have been a high road for emigration and now be impassable; I must therefore first treat this subject in some detail. The change in altitude of the country is an important consideration. A narrow isthmus now separates two marine faunas; if it submerges or has previously submerged, both faunas will flow together or have previously flowed under. On the other hand, where the sea now spreads, formerly dry land may have linked islands or even continents together, enabling land-dwellers to migrate from side to side.DW. FORBES maintains that all the islands of the Atlantic Sea must have been very recently connected with Africa or Europe , just as Europe was connected with America . Other writers have hypothetically bridged each ocean in turn and connected nearly every island to the nearest mainland. And if the arguments of F ORBESconfirmed, one would have to admit that there is scarcely any island which has not yet been connected to a continent. This view cuts the Gordian knot of spreading a species to the remotest points and removes a multitude of difficulties. But, to my best conviction, we are not justified in assuming such prodigious variations within the period of the extant species. It seems to me that we have sufficient evidence of great fluctuations of the soil in our continent, but not of movements so extensive and in such direction that by means of them[p. 389]a connection of Europe with America and the intermediate AtlanticIslands would still result in the current Earth period. On the other hand, I willingly admit the former existence of many islands, now buried in the sea, which have served as resting-places for many species of plants and animals during their migrations. In the coral seas such submerged islands are, in my opinion, still recognizable by the coral rings or atolls standing on them. When it is once fully admitted, as it probably will be some day, that each species had but one place of birth, and when in the course of time we learn something more definite about the means of dispersal, we shall be able to to calculate with some certainty the former extent of the country. On the other hand, I do not believe that it will ever be possible to prove that continents now entirely separate were still in more recent times really or nearly so connected with each other and with the many extant oceanic islands. Some facts of distribution, such as the great diversity of marine fauna on opposite sides of almost every great continent, and a certain degree of relation (to be discussed hereafter) between the range of mammals and the depth of the sea; these and many others seem to me to oppose the assumption of such tremendous geographical upheavals in the recent period as indicated by those of E. F as the great diversity of marine fauna on opposite sides of almost every great continent, and a certain degree of relation (to be discussed hereafter) between the range of mammals and the depth of the sea; these and many others seem to me to oppose the assumption of such tremendous geographical upheavals in the recent period as indicated by those of E. F as the great diversity of marine fauna on opposite sides of almost every great continent, and a certain degree of relation (to be discussed hereafter) between the range of mammals and the depth of the sea; these and many others seem to me to oppose the assumption of such tremendous geographical upheavals in the recent period as indicated by those of E. FORBES established and accepted by many successors views become necessary. The nature and numerical proportions of the inhabitants of oceanic islands likewise seem to me to resist the assumption of an earlier connection with the continents. Nor is their mostly volcanic composition favorable to the assumption that they are mere debris from sunken continents; for if they had been original peaks of mountain ranges on the mainland, at least some of them, like other mountain peaks, would consist of granites, metamorphic slates, strata bearing ancient organic remains, and the like, instead of always being cones of volcanic masses.
I have now a few words to say about the so-called “accidental” means of dissemination, which one would better call “casual.” But I want to confine myself here to the plants. In botanical works one finds remarks that this or that plant is not well suited for wide sowing. But as far as the transport of them through the sea is concerned, it can be asserted that with most of them it is still quite unknown as to the possibility of this. Until the time when I am with Mr. B ERKELEY ‘S Having made a few experiments on it, it was not even known how far seeds are able to resist the injurious influence of salt water. To my astonishment I found that of 87 species, 64 still germinated after lying in sea water for 28 days; and a few still did so after 137 days. It is noteworthy that certain orders are much more attacked by salt water than others. Thus eight out of nine legumes perished, and seven species of the interrelated orders of the Hydrophyllaceae and Polemoniaceae were all dead after a month. For the sake of convenience, I usually only chose small seeds without a pericarp, and since all of them sank after a few days, they naturally cannot navigate large areas of the sea, whether they retain their germinating power in salt water or not. Afterwards I chose larger fruits with capsules, etc., and some of these remained floating for a long time. It is well known how different the buoyancy of a species of wood is when green and when dry. I thought, therefore, that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before Afterwards I chose larger fruits with capsules, etc., and some of these remained floating for a long time. It is well known how different the buoyancy of a species of wood is when green and when dry. I thought, therefore, that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before Afterwards I chose larger fruits with capsules, etc., and some of these remained floating for a long time. It is well known how different the buoyancy of a species of wood is when green and when dry. I thought, therefore, that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before and of these some remained afloat for a long time. It is well known how different the buoyancy of a species of wood is when green and when dry. I thought, therefore, that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before and of these some remained afloat for a long time. It is well known how different the buoyancy of a species of wood is when green and when dry. I thought, therefore, that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before that tides might well carry away plants or their branches, and then cast them ashore, where, once they had dried up, the river might seize them again and carry them to the sea; So I took dry stalks and branches with ripe fruit from 94 kinds of plants and put them in the water. The majority sank at once; but some which green remained on the surface only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before green ones that stayed on the surface for only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before green ones that stayed on the surface for only a very short time now lasted longer. Ripe hazelnuts sank immediately below, but floated if they were before[p. 391]desiccated for 90 days and were still germinating when planted. An asparagus plant with ripe berries floated for 23 days, but after drying it out for 85 days, and its seeds still germinated. The ripe fruits of Helosciadium sank in two days, but after being dried beforehand, they floated and germinated for 90 days. In all, of the 94 dried plants, 18 species swam for 28 days, and some for much longer. So 64 ⁄ 87 = 0.74 of the seed species germinated after 28 days of immersion and 18 ⁄ 94 swam= 0.19 of the dried plant species with ripe seeds (but partly other species than the previous ones) still over 28 days; and therefore, as far as one may infer from these facts, the seeds of 0.14 of the plant species of a region could be carried away by sea-currents for twenty-eight days without detriment to their germinating power. In J OHNSTON ‘ S physical atlas the mean speed of the Atlantic currents is given at 33 sea miles a day (some run 60 m.); and thus, by that mean, those seeds could be carried 924 nautical miles away, and, if then stranded, and immediately blown by the wind to a convenient place farther inland, still germinate.
After me, M ARTINS  made similar experiments, but in a better way, by immersing boxes of seeds in the real sea, so that they were alternately moistened and re-exposed to the air, like real floating plants. He tried ninety-eight species of seeds, mostly different from mine, and including some large fruits, and also seeds from plants which grow near the sea, which probably contributed to the average length of time during which they were floating to hold and to resist the injurious effect of the salt water, to increase something. On the other hand, he did not first dry the fruit with the branches or stalks, which enables some of them[p. 392]would have to swim longer. The result was that 18 ⁄ 98 = 0.185 seed species swam for 42 days and then germinated. I do not doubt, however, that plants which float with the waves keep themselves afloat longer than those which, as in our experiments, are protected from any movement. Therefore, it might be safer to assume that the seeds of about 0.10 species of a flora, once dried, can swim 900 miles of sea and then germinate. The fact that the larger fruits swim longer than the small ones is interesting, as large seeds or fruits cannot well be transported from one region to another except by swimming; hence, like A LPH . ENC ANDOLLE has shown such plants to have restricted ranges.
But seeds can occasionally be propagated in other ways. This is how driftwood gets to most of the islands in the middle of the widest ocean; and the natives of the Coral Islands of the Pacific procure harder stones for their implements almost exclusively from the roots of driftwood trunks; the taxes on these stones form a substantial income for their kings. If irregularly shaped stones are now firmly embedded between the roots of the trees, small patches of earth are also occasionally enclosed behind them, sometimes so precisely that not the slightest bit of it could be washed away during the longest transport. And now I know exactly one case where three dicotyledonous seeds germinated from such a completely enclosed portion of soil between the roots of a 50-year-old oak. Thus I can further show that sometimes dead birds float a long time on the sea without being devoured, and that the seeds contained in their crops retain their germinating power for a long time; peas and vetches e.g. B., which otherwise perish after lying in water for only a few days, showed to my great astonishment that they were still capable of germinating when I took them from the crop of a pigeon that had already been swimming in artificially prepared salt water for 30 days.
Live birds unfailingly play a large part in the transport of live seeds. I could cite many cases to show how often birds of many kinds are carried far across the ocean by storms. We may take it for granted that under such circumstances their speed may often be thirty-five miles an hour, and many writers have quoted it much higher. I have never seen any nutritious species of seed pass through the entrails of a bird, whereas hard seeds and fruits pass unharmed even through the intestines of the grouse. In the course of two months I collected twelve kinds of seeds from the droppings of small birds in my garden, all of which appeared to be good, and some of which I tried actually germinated. However, the following fact is more important. The crop of birds does not secrete any gastric juice and, according to my experiments, does not in the least impair the germinating power of the seeds. Now it is said that when a bird has found and eaten a large quantity of seeds, the grains do not reach the stomach until 12-18 hours. But in that time a bird can easily be carried 500 miles; and when hawks, as they are fond of doing, hunt the tired bird, the contents of its crop may soon be scattered about. Now some hawks and owls swallow their prey whole and after 12-20 hours erupt in bundles of undigested feathers which, as I know from experiments in the Zoological Gardens, often still contain viable seeds. A few seeds of oats, wheat, millet, canary grass, hemp, clover, and chard still germinated, after staying 12-20 hours in the stomachs of various birds of prey, and two chard seeds even grew after being there for two days and fourteen hours. Freshwater fish devour seeds of various terrestrial and aquatic plants; Fish are often eaten by birds, and so those seeds can be scattered from place to place. I put various kinds of seeds in the stomachs of dead fish, and then fed them to pelicans, storks, and ospreys; these birds I put various kinds of seeds in the stomachs of dead fish, and then fed them to pelicans, storks, and ospreys; these birds I put various kinds of seeds in the stomachs of dead fish, and then fed them to pelicans, storks, and ospreys; these birds[p. 394]A few hours later they spit out the seeds in their excrement or broke them out in balls of balls. Many of these seeds then still possessed their power to germinate; others, however, lost them at any time through this process.
Although the beaks and feet of birds are usually quite clean, parts of earth often cling to them. In one case I separated 61 and in another 22 grains of clayey soil from the foot of a partridge, and in this soil was a pebble the size of a vetch seed. Hence, in the same way, seeds may sometimes be carried over great distances, since it can be shown that the soil is everywhere full of seeds. Considering the number of millions of quails that fly across the Mediterranean each year, one cannot doubt the possibility that a few small seeds on their feet may sometimes make their way across or across with them. But I shall come back to this subject later.
As is well known, icebergs are often laden with stones and earth; also brushwood, bones and even a bird’s nest were found on it; hence there can be no doubt that they sometimes, as Lyell has already supposed, carried seeds from one part of the arctic or antarctic zone to another, and in the glacial period even from one part of the present temperate zones to another. Since there is a large number of European plants in the Azores in relation to the other islands of the Atlantic Sea, some of which are closer to the mainland, and (as Mr. HC W ATSONremarked), particularly of species which have a somewhat more northerly character than corresponds to the situation, I suspected that a part of them might have got there with icebergs in the glacial period. At my request, Sir C H . L YELL Mr. H ARTUNG whether he had found erratic blocks on these islands, and received the answer that large blocks of granite and others. rocks that do not occur on the islands. We may therefore safely conclude that icebergs have formerly deposited their burdens on the shores of these mid-oceanic islands,[p. 395]and so it is at least possible that some seeds of northern plants also got there.
Considering that some of the means of transport mentioned above, and others to be discovered later, have been in active use every year for whole centuries and millennia, it would, in my opinion, be a wonderful fact if many plants had not been transported far and wide along these routes . These means of transport are sometimes called accidental, which is not entirely true, as neither the sea currents nor the prevailing direction of the storms are accidental. However, none of these agents is capable of displacing germinable seeds very far away, since the seeds neither retain their germinability for long in sea water, nor can they be transported far into the crops and intestines of the birds. But they suffice occasionally carrying them across seas a few hundred miles wide, and thus conveying them from continent to island, or from island to island, but not from one continent to another. Thus the floras of distant continents will not be greatly mixed in this way, but will remain as widely separated as we now find them. The currents, in their direction, would never yield seeds ofNorth America can bring to Britain , as they wash to our shores from the West Indies , but where, even if they have preserved their vitality all this long way, they cannot endure the climate. Almost every year 1-2 land birds are killed by storms from North America all over the Atlantic Ocean to the Irish and Englishcoast driven; But there was only one way these wanderers could bring seeds with them, and that was in the dirt that happened to be hanging from their feet, which is always in itself a rare coincidence. And even in that case, how small would be the probability that such a seed would get into favorable soil, germinate and reach maturity. Yet it would be a great error to conclude that, being an already well-populated island such as Great Britain , in the last few centuries[p. 396](which, by the way, is difficult to prove) no immigrants from Europe or another continent were taken in by occasional means of transport, even sparsely populated islands, even at greater distances from the mainland, could not receive colonists by such routes. I do not doubt that out of 20 species of seeds or animals swept to an island, though much less populated than Britain, scarcely more than one would be so suited to this new home, now to be naturalized there. But this is, as it seems to me, no great objection to what could be done by such casual means of transport over the long course of geological times, during the uplift and formation of an island, and before it was fully occupied by settlers. In a country that is still almost barren, where as yet no or only a few insects and birds pursue every newly arriving seed grain, it will easily succeed in germinating and surviving if it suits this climate differently.
Dispersal during the Ice Age. ) The concurrence of so many species of plants and animals on mountain heights separated hundreds of miles by lowlands, where the Alpine dwellers cannot advance, is one of the most striking instances of the occurrence of the same species on distant ones Points, with no apparent possibility of migration from one to the other. It is indeed remarkable to see so many species of plants in the snowy regions of the Alps or Pyrenees , and again in the northernmost parts of Europe ; but it is still more remarkable that the plant species of the White Mountains in the United States of Americaall the same as in Labrador , and further, according to A SA G RAY ‘S assurance , are the same as on the highest mountains of Europe . Long ago, in 1747, similar facts led G MELIN to conclude that the same species must have been created independently in different localities, and we might have been inclined to believe that[p. 397]not A GASSIZ et al. drew our attention to the Ice Age, which, as we shall presently see, explains these facts very simply. We have evidence of almost every possible kind, organic and inorganic, that at a very young geological period central Europe and North America suffered from an arctic climate. The ruins of a burned-down house do not tell their story so plainly as the Scottish and Welsh mountains, with their scarred sides, polished faces, floating blocks, tell of the ice-streams with which their valleys were lately filled. The climate in Europe varied so much that inNorthern Italy huge moraines from former glaciers are now planted with mays and vines. Throughout much of the United States , erratic boulders and rocks scarred by drifting ice-bergs and coastal ice, testify with certainty to an earlier period of great cold.
The early influence of the ice climate on the distribution of the inhabitants of Europe , as E DW. FORBES _stated so clearly, is essentially the following. But we will be able to follow the changes more quickly if we assume that a new ice age slowly approaches and then passes and disappears as it did before. To the extent that, with increasing cold, each zone further south in turn becomes more suitable for arctic beings and can no longer appeal to its previous inhabitants, arctic settlers will take the place of the previous ones. At the same time these inhabitants of the temperate regions will in their turn migrate southwards, unless the way is barred to them, in which case they must perish. The mountains will be covered with snow and ice, and the former Alpine dwellers will descend to the plains. When the cold reaches its maximum over time, Europe’s to the south of the Alps and Pyrenees and into Spain . Also the present temperate regions of the United States are being populated with arctic plants and animals, viz[p. 398]nearly the same species as Europe ; for the present inhabitants of the polar countries, who have just been supposed to have migrated everywhere south, are remarkably uniform around the pole. Assuming that the Ice Age started a little earlier or later in North America than in Europe , then the emigration to the south will also start a little earlier or later, but this makes no difference in the final result.
When the warmth returns, the arctic forms retreat northwards again, and the inhabitants of the more temperate regions immediately follow them. When the snow melts at the foot of the mountains, the arctic forms will take possession of the bared and thawed ground; they will climb higher and higher as the heat increases and their brethren on the plain continue the northward retreat. When, therefore, heat is fully restored, the same arctic species which hitherto lived en masse together in the lowlands of the old and new worlds, will be left only on isolated mountain heights and in the arctic zone of both hemispheres.
Thus is understood the convergence of so many plant species in places so immeasurably remote as the mountains of the United States and Europe . Thus further understands the fact that the alpine plants of each mountain range are most closely related to the species just, or almost just, north of them; the migration when the cold sets in and the return migration when the heat returns will generally have been straight south and north. Because the alpine plants of Scotland z. B. are according to HC W ATSON ‘ S remark and those of the Pyrenees according to R AMOND more special with thoserelated to Scandinavia , as those of the United States and the Siberians are more in agreement with the species living in the north of these countries. This view, based on the reliably confirmed course of an earlier Ice Age, seems to me to sufficiently describe the present distribution of the Alpine and[p. 399]arctic species in Europe and North America , that if in still other regions we find like species scattered at distant mountainous heights, we may conclude, without further proof, that a colder climate formerly approached them through intermediate lowlands allowed to wander, which have since become too warm for the same.
If the climate since the Ice Age had ever been somewhat warmer than it is now (as some geologists have inferred from the distribution of fossil Gnathodon shells in the United States ), then the inhabitants of the temperate and frigid zones would still be somewhat remote in very recent times advanced north, only to later withdraw to their present homeland; but I have no sufficient evidence of any such warmer period interspersed after the Ice Age.
The arctic forms will have been exposed to nearly the same climate during their southward migration and return northward, and, also to be noted, have remained together in mass; therefore they would not have been particularly disturbed in their mutual relations and consequently, according to the principles defended in this volume, would not have been exposed to too great a change. However, it would be a little different with our Alpine inhabitants, who, when the heat returned, would have fled from the foot of the mountains higher and higher up their sides up to the summits. For it is not likely that all the same arctic species remained on widely separated mountain ranges and have lived there ever since. Also, those that remain will in all probability have mingled with old Alpine plants, which inhabited the mountains before the Ice Age, and were driven down to the plains for the duration of the coldest period; they will also have been exposed to a somewhat different climatic influence. Their mutual relations may have been somewhat disturbed by this, and they themselves may have become inclined to change; and so it really is. Because if we present the Their mutual relations may have been somewhat disturbed by this, and they themselves may have become inclined to change; and so it really is. Because if we present the Their mutual relations may have been somewhat disturbed by this, and they themselves may have become inclined to change; and so it really is. Because if we present the[p. 400]Comparing Alpine plants and animals from the various large European mountain ranges, we find on the whole many identical species, but some of which appear as varieties, others as doubtful forms, and a few as distinct but closely related or representative species appear.
In explaining what, in my opinion, really took place during the Ice Period, I have suggested that when it began the Arctic organisms around the Pole were as uniform as they are at the present day. But the foregoing remarks apply not only to the severe arctic forms, but also to many subarctic and some north-temperate forms; for some of these species are also common on the lower mountains and plains of North America and Europe, and one may well ask, how do I account for the coincidence of forms which must have taken place in the subarctic and north-temperate zones around the world at the beginning of the Ice Age? At present the forms of the subarctic and north-temperate regions of the old and new worlds are separated from each other by the Atlantic and the northernmost parts of the Pacific Ocean . As during the Ice Age the inhabitants of the old and the new worldliving farther south than now, they must have been more completely separated from one another by further areas of the ocean. I believe that the difficulty mentioned above can be circumvented by looking at even earlier climate changes in an opposite sense. For we have good reason to believe that during the new Pliocene period before the Ice Age, when the majority of the earth’s inhabitants were of the same species as the present ones, the climate was warmer than it is now. We may therefore suppose that organisms which now live below the 60th parallel dwelt in the Pliocene period farther north on the polar circle below the 66°-70° latitude, and that the true arctic beings on the interrupted ones Land-strokes nearer the Poles were restricted.[p. 401] If we now look at a globe we shall find that under the polar circle there is mostly contiguous land from western Europe through Siberia to eastern America . And to this connection of the circumpolar country and the consequent free migration in an already more favorable climate I ascribe the necessary degree of uniformity in the inhabitants of the subarctic and north-temperate zones of the old and new worlds before the Ice Age. (Three preferentially qualified judges, Prof. A SA G RAY , Dr. H OOKER and Prof. O LIVER have joined this view.)
Proceeding from the belief that, as already stated above, our continents remained in almost the same position in relation to one another for a long time, even if they were partly subject to considerable fluctuations in altitude, I have a great inclination to extend the view mentioned even further and to assume that at a still earlier and warmer period, viz., in the older Pliocene period, a large number of the same species of plants and animals inhabited the almost contiguous circumpolar land, and that these plants and animals both in the ancient and in the new worldbegan migrating slowly southward as the climate became cooler, long before the start of the Ice Age. We now see their descendants, I believe, mostly in a modified condition , inhabiting the central parts of Europe and the United States . Proceeding from this assumption, one then understands the relationship, with very little resemblance, between the species of North America and Europe , a relationship which is extremely remarkable given the great distance between the two regions and their separation by the Atlantic Sea . One also understands the curious fact, noticed by some observers, that the natural products of Europe and North America are more closely related during the latter parts of the Tertiary period than they were in the preceding period; for in this warmer period the northern parts of the old and new worlds are more contiguous through intermediate lands[p. 402]connected with each other, but which have since been made useless for emigration due to the cold.
As soon as the jointly emigrant inhabitants of the old and new worlds reached south of the polar circle during the slow temperature decline of the Pliocene period , they were completely cut off from each other. This separation occurred long, long ago, as far as the inhabitants of the more temperate regions are concerned. And when at that time the plant and animal species migrated southward, they are mingled with the natives of the lower latitudes, and in one country American , and in the other EuropeanSpecies have gotten to new competitors. Here, accordingly, everything is done for ample variation of the species, far more than was the case with regard to the polar inhabitants of both parts of the world, who were left cut off on the southern heights of the Alps. It is from this that when we compare the now living produce of more temperate regions of the old and new worlds we find very few identical species (although A SA G RAYrecently shown that their number is greater than previously thought); but we find in every great class many forms, which some naturalists regard as geographical races, and some as distinct species, together with an army of closely related or representative forms, which naturalists regard as distinct species.
In the sea, as on land, a slow southward migration of fauna, which occurred during or a little before the Pliocene period along the continuous coasts of the polar circle, may have been very uniform, according to the theory of modification to explain the many closely related forms, which now live in quite separate areas. With their help, I believe, the existence of a multitude of extant and Tertiary representative species on the eastern and western coasts of the more temperate part of North America can be explained, as well as the far more striking appearance of many closely related crustaceans (in D ANA ‘ s excellent works[p. 403]described), of some fishes and other sea-animals in the Japanese and the Mediterranean at the same time, consequently in regions which are now separated by a large continent and almost a whole hemisphere of equatorial seas.
These cases of kinship, without identity, between the inhabitants of now separate seas, as between the former and present inhabitants of the temperate countries of North America and Europe , are inexplicable from the theory of creation. We cannot say that they were similarly made to adapt to the similar natural conditions of the two countries; because if we z. For example, if we compare certain parts of South America with the southern continents of the old world , we find traits in both which correspond exactly in their natural constitution, but are quite unlike in their inhabitants.
However, we must return to our immediate task, which is Ice Age. I am convinced that E DW . FORBES ‘ theory is capable of great extension . In Europe we have the clearest evidence of a cold period from the west coasts of Britain eastward to the Ural chain and southward to the Pyrenees . From the mammals frozen in the ice and the condition of the mountain vegetation, Siberia had been affected in a similar way. Along the Himalayashave glaciers, at points 900 English miles distant from each other, left traces of their former great depths; and in Sikkim Dr. H OOKER Mays grow on old giant moraines. To the south of the equator we have some direct evidence of former glacial activity in New Zealand , and the reappearance of the same species of plants on widely separated mountains of that island speaks for the same story. According to the information given to me by the indefatigable geologist WB C LARKE , clear traces of former glacial activity also appear to occur in the south-eastern tip of New Holland .
Let’s look around America . In the northern half, rock debris transported by ice has been observed at[p. 404]down the east side to 36° and on the coast of the Pacific , where the climate is now so different, to 46° north latitude; erratic blocks have also been seen in the Rocky Mountains . In the Cordilleras of equatorial South America , glaciers have formerly descended far beyond their present limits. At Central Chili I have examined a prodigious heap of detritus queerly interspersing the Portillo valley , and, as I am now convinced, a vast moraine deep beneath every glacier now extant there. D. F ORBES has given me the following information, now more precise: that he was in theCordillera from 13° to 30° SBr. found, at about 12000’, heavily grooved rocks much like those he saw in Norway, and large masses of detritus with grooved boulders; along this whole stretch of the Cordillera there are no real glaciers, even at much higher altitudes. Farther south on both sides of the continent, from latitude 41° to the southern tip, we find the clearest evidence of former glacial activity in mighty boulders carried far from their birthplace.
We do not know whether the Ice Age was exactly simultaneous at all these points on quite opposite sides of the earth; but it fell, as can be proved in almost all cases, into the last geological period. In the same way we have excellent evidence that they have everywhere been of immense duration in terms of years. It may have begun or ended earlier in one part of the world than in another; but as it lasted a long time everywhere, and was everywhere simultaneous, at least in a geological sense, it seems to me probable that at least a part of the glacial events coincided exactly in time in all these places all over the world. Therefore, unless we have some definite proof to the contrary, we may supposeNorth America , in the equatorial cordilleras of the tropical and warm-temperate zones, and on either side of the southern extremity of this continent. Is this to be accepted[p. 405]allowed, one must also assume that the temperature of the whole earth was simultaneously cooler during this period; but it will suffice for my purpose if the temperature was simultaneously lower only over certain broad north-south stretches of the earth.
Proceeding from this assumption, that the earth, or at least broad meridianal strips of it, have simultaneously grown colder from one pole to the other, much light can be given as to the present distribution of identical and allied species. dr HOOKER has shown that in America 40-50 flowering plants of Tierra del Fuego , forming no inconsiderable part of the small flora there, in spite of the immense distance between the two points, agree with European species; there are also many closely related species. A multitude of peculiar species from European genera occur on the high mountains of tropical America . On the highest mountains A few European genera of G ARDENER have been found in Brazil , which do not get along in the far-stretched warm intermediate countries. At the Silla of Caraccas , A L . VON H UMBOLDT long ago clans characteristic of the Cordillera . Various European forms and a few representative species of the peculiar flora of the Cape of Good Hope occur on the Abyssinian Mountains . At the Cap are a few EuropeansSpecies not thought to be introduced, and various representative forms of European species have been found on the mountains, such as have not yet been discovered in the tropical countries of Africa . dr H OOKER has lately shown that several of the plants growing on Fernando Po Island in the Gulf of Guinea are closely related to those of the Abyssinian Mountains on the other side of the African continent, and to those of temperate Europe ; This is one of the most startling facts in plant geography. — On the Himalayas and on the isolated mountain ranges of theOn the Indian Peninsula, on the heights of Ceylon , and on the volcanic cones of Java , many plants occur which are either of the species[p. 406]in accordance with one another, or mutually representing one another and at the same time vicarious for European forms, but are not found in the intermediate warm lowlands. A register of the clans collected on the lofty mountain-tops of Java gives a picture as of a collection made on European mountains. Still more striking is the fact that the South Australian forms are apparently represented by plants growing on the mountain heights of Borneo. Some of these Australian ( New Dutch ) forms extend after Dr. H OOKER along the heights of the Malacca Peninsula and are thinly scattered over India on the one hand, and northward to Japan on the other .
On the southern mountains of New Holland , Dr. F. M ÜLLER discovered several European species; other non-human introduced species occur in the lowlands and, as Dr. H OOKER says a long list of European taxa could be drawn up, found in New Holland but not in the hot intermediate countries. In the excellent introduction to the flora of New Zealand , Dr. HOOKER _still other analogous and striking instances as to the plants of this great island. We see, therefore, that over the whole surface of the earth, on the one hand, the plants growing on the higher mountains, and on the other hand, those widespread in the temperate lowlands of the northern and southern hemispheres, are sometimes of the same kind; but more often they appear specifically different, though oddly related.
This brief outline applies only to plants alone; but exactly analogous facts may be adduced concerning the distribution of land animals. Similar cases also occur in sea animals. I will cite in evidence the remark of one of the best sources, namely Professor D ANA , “that it is certainly a wonderful fact that New Zealand bears, in its crustaceans, a greater affinity to its antipode Great Britain than to any other part of the world.” So speaks Sir J. R ICHARDSON of[p. 407]the reappearance of Nordic fish forms on the coasts of New Zealand , Tasmania , etc. Dr. H OOKER tells me that New Zealand shares 25 species of algae with Europe that have not yet been found in the intermediate tropical seas.
It is to be observed that the northern species and forms found in the southern parts of the southern hemisphere and on the tropical high mountains are not arctic, but correspond to the northern part of the temperate zone. Mr. HC W ATSON recently remarked that “as one advances from polar to tropical latitudes, the alpine or mountainous forms of organisms become less arctic.” of so dubious a value that it has been regarded by some naturalists as essentially different from European ones species and called by others mere varieties. But some of them are certainly identical and many, even if closely related to Nordic forms, must be recognized as distinct species.
Let us now see what information the foregoing facts may afford as to the assumption, supported by a great deal of geological evidence, that the whole surface of the earth, or at least a large part of it, was at the same time much colder during the Ice Age than it is now. The ice period must have been very long in terms of years; and if we consider over what wide areas some naturalized plants and animals have spread in a few centuries, this period could have sufficed for any migration, however far. As the cold increased only slowly, all the tropical plants and animals would have retreated towards the equator on either side, followed by the inhabitants of temperate regions, followed by those of the polar zones; but we are not concerned with the latter at this moment. The task is an extremely complicated one. Even the Pleistocene equatorial flora, probably present before the Ice Age,[p. 408]which would have corresponded to an even more than tropical climate, must not be completely ignored. This ancient equatorial flora would have been superseded by the present equatorial flora during the Ice Age, and the two Pleistocene subtropical floras now mingled and fused in number. Likewise, during the ice age, very large changes in the moisture and a. climatic conditions may have occurred, as a result of which some animals and plants would have migrated in different proportions. All the conditions of life would thus have undergone simultaneous and significant changes during the glacial period in the tropics. Many of the tropical organisms undoubtedly became extinct in the process; how many, no one can say. Perhaps the tropical regions used to be as rich in species as they are nowCape of Good Hope and some temperate parts of New Holland .
As we know that many tropical plants and animals can withstand a fair degree of cold, some of them may have escaped destruction by a moderate decrease in temperature, especially if they have escaped into the deepest, most sheltered and warmest regions. But the main thing that must not be forgotten is that all tropical products must have suffered to a greater or lesser extent. It is most difficult to say how they escaped utter extinction; the possibility of some acclimatization during the very slow approach of the cold period must not be entirely overlooked. On the other hand, the inhabitants of temperate regions, which could draw closer to the equator, were placed in somewhat new conditions, but suffered less. It is also certain that many plants of temperate climates, when protected from competition, can endure a much warmer climate than their proper climate. Hence it seems to me possible that, while the tropical produce was in a state of disrepair, and could not offer serious resistance to the invaders, a certain number of the most vigorous and dominant forms[p. 409]entered the ranks of the natives in the temperate zone, and have reached and even crossed the equator. The incursion was generally favored by highlands and perhaps an arid climate; because dr F ALCONER tells me that it is the dampness associated with the heat of the tropics which becomes so pernicious to the perennial plants of more temperate regions. On the other hand, the wettest and warmest regions will have served as places of refuge for the natives of the tropics. The mountain ranges of the northwestern Himalayas and the long cordillera range appear to have formed two great lines of invasion; and it is a striking fact that, according to Dr. HOOKER ‘S _ _According to the last communication, the 46 flowering plants which Tierra del Fuego has in common with Europe all also occur in North America , which must have been on their march route. If one were to conclude from this that the land in some tropical regions was higher than it is now when the organisms coming from temperate regions migrated through it, then at least we lack all the evidence for this. Therefore I am forced to assume that some inhabitants of the temperate zones even penetrated into the lowlands of the tropics and especially East India and crossed them when, at the time of the greatest cold, arctic forms migrated from their homeland 25 degrees of latitude southward and entered the country on the foot of the Pyreneescovered. During this period of the greatest cold, the climate below the equator at sea level would have been about the same as it is now there at 5000′-6000’ above sea level. In this period of greatest cold, vast spaces in the tropical lowlands were, in my opinion, covered with a vegetation composed of forms of tropical and temperate regions comparable to those which, according to H OOKER ‘ S vivid description, now abound in wondrous luxuriance at the foot of the Himalayas in 4000 ′–5000′ above sea level deployed.
When M ANN was botanizing on the island of Fernando Po , he saw individual plant forms from temperate Europe appearing from 5000’ above, and Dr. S EEMANN found in the mountains of[p. 410] Panama at 2000′ a vegetation like in Mexico mixed harmoniously with forms of the hottest zone and those of the temperate one; hence the possibility arises that under certain climatic conditions genuine tropical plants may live for an indefinite period with forms of temperate climates.
I had hoped for a time to find evidence that somewhere on earth the tropical regions had been spared the freezing effects of the Ice Period and offered a safe haven to the suffering tropical dwellers. We cannot seek this refuge in the East Indies or in Ceylon , as forms of temperate climates have nearly all attained their discrete mountain heights; we cannot find them in the Malay Archipelago, for we see European forms appearing on the volcanic cones of Java , and products of the temperate part of New Holland on the heights of Borneo . InAfrica not only have some temperate European forms traversed Abyssinia on the west side to its southern extremity, but also forms of temperate climate from there up the continent to Fernando Pocrossed on the west side, perhaps with the help of mountain ranges, which according to some indications traverse the mainland from east to west. But if it were also supposed that some extensive tropical region had retained its full warmth during the Ice Age, this supposition would not help us much, because the tropical forms which have survived therein have not, in so short a period of time, well differed from any of these large tropical areas could have migrated. Nor have the tropical forms of the whole surface of the earth held against each other such a uniform appearance as if they had set out from a common haven of safety.
The eastern plains of tropical South America appear to have suffered least from the Ice Age; and yet here, too, a few forms of temperate regions have been found in the mountains of Brazil , which cover the continent[p. 411]must have crossed from the Cordilleras ; and likewise during the same period there seems to have been a migration from the Cordilleras to Caraccas . But now B ATES , who with great zeal has studied the insect fauna of the Guiana Amazonarea, recently vehemently opposed any assumption of a recent cooling of this large area, showing that it is rich in very peculiar native butterfly forms, which evidently support the supposition of a recent extinction in the vicinity of the contradict the equator. However, I will not presume to say to what extent this phenomenon can be explained by assuming an almost complete extinction of a Pleistocene fauna in the Ice Age and the formation of the present equatorial fauna by the combination of the two previous subtropical faunas.
Thus, I believe, during the glacial period, a considerable number of plants, some terrestrial animals, and various sea-dwellers entered the tropics from both temperate zones, and some even crossed the equator. When the warmth returned, the forms emanating from the temperate climates naturally ascended the mountains and disappeared from the lowlands; those who had not reached the equator returned north and south to their former homes; but those chiefly northern forms, which had already crossed the equator, migrated further into the temperate latitudes of the opposite hemisphere. Although geological research shows that that the whole mass of arctic conchylia has scarcely undergone any essential modification on their long migration south and their return migration north, the proportion is quite different with regard to the intruding forms which are to be found on the tropical mountains and in the southern hemisphere set. Surrounded by strangers they came into competition with many new forms of life; and it is probable that variations in structure, organic action, and habits have resulted and[p. 412]have been trained through natural breeding. Thus many of these migrants, though evidently still related to their brethren in the other hemisphere, live on in their new homes as distinguished varieties or distinct species.
It is a curious fact, insisted on by H OOKER as to America , and A LPHONS D E C ANDOLLE as to Australia , that many more identical and related plants appear to have migrated from north to south than vice versa. We therefore see few southern plant-forms on the mountains of Borneo and Abyssinia. I suspect that this predominant migration from north to south is due to the greater extent of land in the north, and the numerous existence of the northern forms in their native land, as a result of which, by natural selection and more varied competition, they have already attained a higher perfection and dominion than the more southern forms had arrived. And now that both were mingled during the glacial period, the northern forms were better suited to overcoming the southern ones—as we still see the European immigrants to-day covering the soil of La Plata and, for the last 30-40 years, of New Holland . The Neil-gherrie Mountains in East Indiaoffer a partial exception, however, in that, as Dr. H OOKER says AustralianForms there rapidly naturalize and spread by seed. Before the Ice Age these tropical mountains were undoubtedly populated with native alpine plants. In many islands the native produce has already been equaled or surpassed in quantity by the naturalized; and even if those first ones have not yet been ousted, their number has already greatly decreased, and this is the first step towards the end. A mountain is an island on land, and the tropical mountains before the Ice Age must have been entirely isolated. I believe that the produce of these islands has yielded inland to that of the greater Nordic stretches in much the same way as the produce of the[p. 413]Islands in the sea have finally been everywhere displaced by those naturalized there by man.
I am far from believing that all the difficulties have already been settled by the view here given of the distribution and relations of the allied species which dwell in the northern and southern temperate zones and on the mountains of the tropics. It is very difficult to conceive how so large a number of peculiar forms, confined to the tropics, could have survived the coldest part of the Ice Age. The number of forms in New Holland associated with forms of temperate Europe related, but yet so different from them that it is impossible to believe in a modification since the glacial period, perhaps indicates a still older cold period, which is related to the speculations of some modern geologists. — The exact directions and means of migration, or the causes why some species have migrated and not others, or why certain species have undergone modification and been used in the formation of new groups of forms, while others have remained unaltered, cannot be ascertained prove. We cannot hope to explain such relations until we can say why one species and not the other can be naturalized by human activity in foreign lands, or why the one is twice or thrice so widespread that
I have said that many difficulties remain to be overcome. Some of the strangest, Dr. H OOKER in his botanical works on the Antarctic regions with admirable clarity. These cannot be discussed here. I will only remark that when dealing with the occurrence of a species at points so immensely distant as Kerguelen Land , New Zealand , and Tierra del Fuego , in my opinion (as also L YELL supposes ) icebergs towards the ends of the ice time in itself[p. 414]may have played a large part in their dissemination. But the occurrence of a few distinct species from far southern genera at this or that remote point of the southern peninsula is, on my modified-reproduction theory, a far more remarkable difficult example. For some of these species are so divergent that it cannot be supposed that the time from the beginning of the Ice Age up to now could have sufficed for their migration and subsequent modification to the required level. These facts seem to me to indicate that very different peculiar species radiate from some common center; and I am inclined to look around for a warmer period before the Ice Age in both the southern and northern hemispheres, where the Antarctic lands, now covered with ice, have possessed a flora peculiar and distinct. I suspect that even before the extinction of this flora by the Ice Age, a few forms of it had spread, by occasional means of transport, to various remote points in the southern hemisphere. In doing so, they may have served as resting places on some islands that were either still there or had already sunk. And so, I believe, have the southern coasts of In doing so, they may have served as resting places on some islands that were either still there or had already sunk. And so, I believe, have the southern coasts of In doing so, they may have served as resting places on some islands that were either still there or had already sunk. And so, I believe, have the southern coasts ofAmerica , New Holland , and New Zealand are similarly colored by the same peculiar forms of plant life.
Sir CH . Lyell , in a manner almost like mine, has speculated on the influence of great fluctuations in climate on the geographical distribution of life-forms . I believe, then, that the surface of the earth has lately experienced one of these great cycles, and that this supposition, together with the assumption of modification by natural selection, explains a multitude of facts in the present distribution of life-forms, both identical and allied leaves. The streams of life might be said to have flowed for a short time from north and south, and crossed the equator;[p. 415]but those from the north were so much stronger that they flooded the south. As the tides leave their drift in horizontal lines deposited on the shore, but rise to different heights on different shores, so different life-currents have left their lively drift on our mountain heights, ranging from the arctic lowlands to great equatorial heights slowly rising line. The various creatures left behind on the beach can be compared with wild human races, which are almost everywhere pushed back and are still preserved in mountain fortresses as interesting remains of the former populations of the surrounding lowlands.