Chapter 19 Chapter 12 Geographic Distribution

The present-day distribution cannot be explained by differences in physical conditions--Importance of obstacles--Affinity of organisms on the same continent--Centers of creation--Methods of dispersal due to variations of climate, elevation of land, and chance causes - Dispersion in Glacial Ages - Alternation of Southern and Northern Ice Ages. When we consider the distribution of life on the surface of the earth, the first great thing that strikes us is that neither the likeness nor the dissimilarity of life in every place can be entirely explained by climatic and other physical conditions.Nearly every author who has studied this problem in recent times has come to this conclusion.The case of the Americas alone is almost sufficient to justify this conclusion; for, if the arctic and northern temperate regions are excluded, all authors agree that the distinction between the "New World" and the "Old World" is one of geographical distribution. However, if we travel across the vast continent of America, from the central region of the United States to its southernmost tip, we will encounter an enormous variety of physical conditions: wet regions, dry deserts, towering Mountains, grasslands, forests, swamps, lakes and rivers, these places are all under various temperatures. There is hardly a climatic or external condition in the "Old World" that cannot be paralleled with that of the "New World"—at least as closely as the same species generally requires.No doubt it may be pointed out that there are pockets of the "Old World" which are hotter than anywhere in the "New World," but the fauna which inhabit such places are not different from those which surround them; Phenomena in small areas with slightly special conditions are still rare.In spite of this general parallelism between the conditions of the "Old World" and the "New World," how different are their organisms!

In the southern hemisphere, if we compare the large land masses of Australia, South Africa, and the western part of South America between the latitudes of twenty-fifth and thirty-fifth degrees, we shall see some places which are extremely similar in all conditions. , however, it is probably impossible to point to three fauna and flora as strictly different as the fauna and flora of the three continents.Let us then compare the organisms of South America south of the thirty-fifth latitude with those north of the twenty-fifth. There is a space of ten degrees between the two places, and under quite different conditions; are incomparably more closely related to one another than they are to the productions of Australia or Africa of a like climate.Similar facts may be cited with regard to marine organisms.

The second thing which strikes us in our general observations, is the close and important relation which hinders free migration of any kind, to the differences of the productions of different regions.We can see this in the great difference between nearly all the terrestrial organisms of the old and new worlds, except in the northern regions, where the lands are nearly all connected, and where the climate differs very little, and the types of the northern temperate regions are, Presumably freedom of movement, as strictly arctic creatures do at present, is possible.We see the same fact in the great differences between the organisms of Australia, Africa, and South America at the same latitude: for the isolation of these places from one another has almost reached its zenith.We see the same fact on the various continents; for on either side of great continuous mountains, great deserts, and even great rivers, we see different creatures; The oceans are not so impassable, or so long lasting, that the organisms on the same continent differ in a much less degree than those on different continents.

We can see the same law with regard to the ocean.The marine life of the east and west coasts of South America is very different, except that very few shellfish, crustaceans, and echinoderms are identical; Thirty-thirds of the fish are identical; and this fact has led naturalists to believe that this isthmus was formerly the surface of the sea.To the west of the coast of America stretches a wide and boundless sea, without an island where the pilgrim can rest; Completely different fauna.The three marine fauna, therefore, in the same climate, form parallel lines not far from each other, and are distributed far to the north and south; but separated by such obstacles as impassable land or sea, the three The species fauna are almost completely different.On the other hand, going further westward from the eastern islands of the tropical Pacific Ocean, we no longer encounter impassable obstacles, and there are innumerable islands, or continuous coasts, which may serve as stopping places, after a journey of one hemisphere. Afterwards, we reach the coast of Africa; in this vast space, we will not encounter very different marine fauna.

Although only a few marine animals are common to the above-mentioned three closely related fauna of the eastern, western, and eastern Pacific islands, there are still many fishes that range from the Pacific to the Indian Ocean, and on almost completely opposite meridians. There are also many common shellfish on the eastern Pacific Islands and the eastern coast of Africa. The third great event, which has been partly included in the foregoing description, is the affinity of the organic beings on the same continent or in the same sea, though the species themselves differ in different places and places.This is a law of the widest generality, and every continent furnishes innumerable examples.Yet when the naturalist travels, say, from north to south, he will encounter the succession of successive groups of organisms, closely allied but distinct in species, which must impress them.He will hear nearly alike songs of closely allied and distinct species of birds, and he will see that their nests, though not identical, are of a like construction, and that the eggs therein are of nearly the same colour.On the plains near the Straits of Magellan there inhabits one species of Rhea [the American ostrich], and north of the plain of La Plata another species of the same genus inhabits; Ostrich or emu (emu).On the same plain of La Plata we find the agouti and the bizcacha, animals which have almost the same habits as the European mountain and domestic hares, and which belong to the same order of rodents. , but their structure clearly presents an American pattern.We ascend the majestic Cordillera and see an alpine species of chinchilla; we gaze at rivers and see no beaver or musk-rat but coypu and capybara (capybara), these are rodent orders belonging to the South American model.There are countless other examples that could be cited.If we look at the islands off the coast of America, however vastly different their geological formations may be, the creatures that inhabit them are essentially of the American pattern, though they may all be peculiar species.As in the previous chapter, we can look back at past ages, and we will see that American-type organisms predominated both on the continent and in the seas of America.In such facts we see a certain deep organic connection through space and time, throughout the same region of land and water, and independent of physical conditions.The naturalist must be insensitive if he does not wish to inquire into what this connection is.

This connection is merely hereditary, and this cause alone, so far as we know with certainty, makes living beings very much like each other, or, as we see in varieties, nearly like each other.The dissimilarity of the organisms of different regions may be ascribed to changes through variation and natural selection, and, in a second, presumably to the certain influence of different physical conditions.The degree of dissimilarity depends on the migration of the more dominant forms from one place to another being more or less effectively hindered during a considerable remote period; The nature and number of living beings,--and depending on the preservation of the different variations brought about by the interaction of living beings; and in the struggle of life living beings and the connection of living beings, as I have often said before, are the most important of all relations relation.Obstacles thus assume a high importance by preventing migration.Just as time plays a role in the slow process of mutation through natural selection.Species that are widespread, numerous, and have already outmaneuvered many competitors in their widespread home areas, have the best chance of acquiring new positions when they expand into new places.In their new home they will encounter new conditions, and will often undergo further modifications and improvements, thus gaining further victories and producing hordes of modified offspring.On this principle of descent with variation, we can understand why a part of a genus, a whole genus, or even a family are so generally and markedly confined to one place.

As stated in the previous chapter, there is no evidence for the existence of any law of necessary development.As the variability of each species has its own independent character, and the variability can be exploited by natural selection only when it benefits each individual in the complex struggle of life, the amount of variation will not be the same in different species.If several species, after long competition with each other in their native country, migrate collectively into a new and later isolated country, they will seldom vary; for removal and isolation themselves produce no effect.These factors come into play only so far as they bring living things into new relations with each other, and to a lesser extent with the physical conditions around them.As we have seen in the preceding chapter, some forms of life have retained nearly the same character from a very remote geological age, so that certain species have migrated over vast spaces without great changes, or at all. No change at all.

On this view, the several species of the same genus, though inhabiting very widely separated parts of the world, must have originally occurred in the same country of origin, as they are all descended from the same ancestor.As for those species which have changed little throughout geological time, it is not difficult to believe that they have all migrated from the same place; for during periods of great geographical and climatic changes which have occurred in succession since ancient times, almost any migration It's all possible.But there are great difficulties in explaining the many other cases in which we have reason to believe that the species of the same genus have arisen at relatively recent times.It is also evident that individuals of the same species, though at present inhabiting distant and isolated places, must have descended from the place where their parents first arose, since, as has been shown, identical individuals arose from parents of different species. is unbelievable.

Supposed Single Center of Creation—We now come to a question which has been discussed at length by naturalists, namely, whether species were created in one place or in many places on the surface of the earth.How the same species migrated from one place to so many distant and isolated places as we see it to-day is, no doubt, extremely difficult to understand.But whoever rejects the simplicity of the view that each species first arose in one place is captivating, rejects the real cause of common occurrence and subsequent migrations, and puts the work of miracles Bring in.It is generally admitted that in most cases the habitats of a species are always continuous; if a plant or animal inhabits two places which are far apart, or which have an intermediate zone not easily passed by in migration , then such a thing is considered a noteworthy exception.The impossibility of passage through the sea in migration is probably more evident in terrestrial mammals than in any other organic beings: we have therefore not yet seen a case where the same mammal inhabits places so widely separated that it cannot be explained.No geologist finds it difficult to explain that Great Britain has the same quadrupeds as the rest of Europe, for those parts were once united.But if the same species could arise in two separate places, why don't we see a mammal common to Europe and Australia or South America?The conditions of life are nearly the same, so that many of the animals and plants of Europe have become naturalized in America and Australia; and at such great distances in the northern and southern hemispheres there are several identical native plants.The answer, so far as I believe, is that certain plants, by their various methods of dispersal, have passed in their migrations through broad and disconnected intermediate zones, which mammals cannot migrate.The great and marked influence of obstacles of all kinds can be explained only on the view that the majority of the species arose on one side of the obstacle, and were unable to migrate to the opposite side.A few families, many subfamilies, many genera, and divisions of a greater number of genera, are confined to a single place; and several naturalists have observed that the most natural genera, that is, those whose species are most closely related to each other, generally are all confined to the same place, and if they are widely distributed, their distribution is continuous.What a strange anomaly it would be if, when we went one step further down in the series, down to individuals of the same species, there should be a diametrically opposed law governing them, which, at least initially, were not confined to one place. what!

It seems to me, then, as many other naturalists have thought, that each species arose in only one place, and thereafter, by such forces as its migration and subsistence, under past and present conditions, would permit, from there This is the most likely point of view.No doubt in many cases we cannot explain how the same species could have been moved from one point to another, but geographical and meteorological changes which have certainly occurred in recent geological times must have distorted the former continuous distribution of many species. It's not continuous.We must therefore inquire whether the exceptions to the continuity of distribution are so numerous, and of such a serious nature, that we should give up the belief, which on general consideration seems possible—that the individual species are in the same within the region and migrated from there as much as possible.It would be tiresome to discuss all the exceptions of the same species which now live in isolated points far apart, and I never dared to claim that I could give any explanation for many instances.But, after a few introductions, I shall present my discussion of a few of the most remarkable facts; The wide distribution of freshwater organisms (discussed in the next chapter); thirdly, the occurrence of the same terrestrial species on islands and their nearest continents, though separated by hundreds of miles of sea.If the existence of the same species at widely separated and isolated points on the surface of the earth can in many instances be explained from the view that individual lines of species migrated from a single In view of the ignorance and ignorance of various geographical and geographical changes and various temporary methods of transportation, I think it is incomparably safest to believe that single origin is the law.

In discussing this question, we must at the same time consider a matter of equal importance to us, namely, whether the several species of a genus (which, according to our theory, must all descend from a common ancestor) can be derived from a region. Migrate, and mutate as they migrate.Where the majority of species inhabiting one country are closely allied with, but not quite alike with, those of another, if it can be shown that their migration from one country to another probably took place at some former age, Our general view would then be more solid; for the explanation of this phenomenon is evident on the principle of descent with variation.For example, a volcanic island raised and formed a few hundred miles from the continent would probably in time receive a few organisms from the continent, and their descendants, though modified, would still, by heredity, resemble those of the continent. creatures are related.Situations of this nature are general, and, as we shall see hereafter, cannot be explained by the theory of independent creation.The view that species in one area are related to those in another is not very different from that maintained by Mr. Wallace, who asserts that "the generation of individual species, in space and in relation to previously existing closely allied species, It's the same in time."It is now clear that he ascribed this unity to evolution with variation. The question of whether the center of creation is single or multiple differs from another close question—whether all individuals of the same species are descended from a pair of mates, or from a single hermaphroditic individual , or, as some authors suppose, descended from many simultaneously created individuals.As regards organic beings which never interbreed, if they exist, the individual species must have been descended from successively modified varieties, which have at one time repelled each other, but never intermingled with other individuals or varieties of the same species; At each successive stage of variation, all individuals of the same type are descended from a single parent.But in most cases, that is, with respect to all organic beings which are customarily mated at each birth, and which are occasionally crossed, the individuals of the same species in the same country will, by crossing each other, remain more or less the same; many individuals will vary simultaneously. , and the full amount of variation at each stage will not be descended from only a single parent.To illustrate what I mean by an example: English racehorses are not like every other breed of horse, but their differences and advantages are not inherited from any one pair of parents alone, but are due to the fact that in each The careful selection and training of many individuals continues through the generations. Before discussing the three classes of facts which I have selected above as the most difficult problems of the "single center of creation" doctrine, I must say a little something about the method of diffusion. method of spreading Sir Lyall and other authors have discussed this issue well.I can only cite here some of the more important facts and their simplest summary.Climate change must have had a powerful effect on migration.A place which today, by virtue of the nature of the climate, is impassable to certain creatures, would have probably been a great migratory road in former times, when the climate was different.This aspect will now be discussed in somewhat more detail.Changes in the level of the land must have also had an important influence: for instance, a narrow isthmus now separates two marine fauna; Fauna would mix, or have mixed before.Where the oceans are today, there may have been lands in previous ages connecting islands, and possibly continents, so that terrestrial life could escape from here to elsewhere.No geologist has disputed that great changes in the level of land have occurred during the existence of present-day organisms.Forbes maintains that all the islands of the Atlantic must have been connected in the recent past with either Europe or Africa, and that Europe was likewise connected with America.Other writers have thus supposed that every sea was accessible by land, and that nearly every island was connected with some continent.If Forbes' thesis is to be believed, it must be admitted that in the recent past there was scarcely an island which was not connected with a continent.This point of view solves the problem of the distribution of the same species over vastly distant points, and eliminates many difficulties; but, so far as I can judge, we cannot be permitted to admit that species exist There have been such great geographical changes in the period of time.It seems to me that we have abundant evidence of great variations in the level of land or of seas; but no evidence that our continents have ever varied so greatly in position and extent that they have been compared to each other in modern times. Connected, and connected with several intervening ocean islands.I frankly admit that there were formerly many islands now submerged in the sea, which probably formerly served as resting-places for migrating plants and animals.In coral-forming seas there were sunken islands of this kind, and on them are now rings of coral, the mark of atolls.It will some day be admitted that the individual species have descended from a single place, and when this has been sufficiently admitted, and in the course of time, when we know something certain about the method of distribution, we can safely The extent of the former land has been inferred.But I do not believe it will be possible to prove in the future that many of the continents that are now very separate were connected, or nearly connected, in modern times, and with many extant oceanic islands.Certain facts of distribution,--such as the great difference in marine fauna on either side of almost every continent,--the close relation of several terrestrial and even marine Tertiary The degree to which the mammals inhabiting the ocean resemble those of the adjacent continents depends in part on the intervening ocean depths (to be recounted later)—this and other such facts are in contrast to the fact that in recent times there have been great geographical changes. The opposite is true of what Forbes has said and which is acknowledged by his followers.The nature and relative proportions of the sea-island organisms are likewise contrary to the belief that sea-islands were once connected to the mainland.Moreover, the almost universal volcanic element of these islands does not support the claim that they are all remnants of the sinking of the continent;—if they had originally existed as mountains of the continent, at least some of them would have been like other mountains. That ground is composed of granite, metamorphic schist, ancient fossil rocks, and other rocks, not just the accumulation of volcanic materials. Now I must say a few words about what is called the method of accident, which is more properly called the method of distribution of chance.I'm just talking about plants here.In botanical writings it is often said that this or that plant is unsuitable for widespread dissemination; but the ease or ease of transport across the seas is, so to speak, almost entirely unknown.It was not even known how resistant the seeds were to the damaging effects of sea water, till Mr. Berkeley helped me to perform several experiments.I was surprised to find that out of 87 kinds of seeds, 64 kinds can still germinate after soaking for 28 days, and a few can survive after soaking for 137 days.It is worth noting that some orders were far more damaged than others: nine species of pod plants were tested, and all but one were poorly resistant to salt water; ) and seven species of the family Polemoniaceae died when soaked for a month.For convenience, I chiefly experimented with small seeds, without capsule or pulp; and as these sank after a few days, they could not float across wide seas, whether they would be damaged by sea water or not.Afterwards I experimented with some larger fruits, pods, etc., some of which were able to float for a long time.What a difference in buoyancy is known between fresh wood and dry wood; and I have seen great waters often carry overboard dry plants or twigs bearing gourds or fruits.This thought, therefore, led me to dry the stems and branches bearing ripe fruits of ninety-four species of plants, and place them in sea water.Most sink quickly, but some float for a short time when fresh and for a long time when dry; for example, ripe hazelnuts sink instantly but float 90% when dry days, and these seeds can later germinate; asparagus with mature berries floats for 23 days, but after drying it can float for 85 days, and these seeds can germinate later; ) mature seeds sink in two days, and can float for about 90 days after drying, and will germinate in the future.In total, 18 of the 94 dried plants were able to float for about 28 days; and some of the 18 could float longer.That is, of 87 species of seeds, 64 species germinated after soaking in water for 28 days; ), eighteen of which float for about twenty-eight days; so, if any inference can be drawn from these meager facts, we may assert that of the seeds of 100 species of plants anywhere, 14 species of Seeds will float for about 28 days and will retain their germination power.Johnsten's "Geographic Map" shows some Atlantic currents with an average speed of 33 miles a day (and some with a speed of 60 miles a day); The seeds of fourteen species may have drifted across 924 miles of sea to another place, and would probably have germinated if a land-wind had blown them to a suitable spot after stranding. After these experiments by me, M. Mariens carried out similar experiments, but in a better way, for he put the seeds in a box and let them float on the sea, so that the seeds were sometimes Wet and sometimes exposed to the air, like real floating plants.He experimented with 98 species of seeds, most of which were different from mine; but he selected the seeds of many large fruits and seaside plants; this presumably prolongs their buoyancy and increases their resistance to the damaging effects of seawater. .On the other hand, he did not first dry the fruit-bearing plants or branches; and drying, as we have said, keeps certain plants afloat longer.It turned out that seeds of 18 of 98 different species of plants floated for 42 days and were able to germinate later.But I do not doubt that the plants exposed to the waves have a shorter float time than the plants in our experiments which were not affected by vigorous exercise.It may therefore be safer to assume that the seeds of about 10 of the 100 species of plants in a flora, when dried, can float across a sea about 900 miles wide, and thereafter remain germination.The fact that large fruits often float longer than small ones is interesting; for plants with large seeds or large fruits are generally, as Condor says, limited in range, and they are very Difficult to convey by any other method.Seeds can sometimes be delivered by another method.Drifting timbers are washed up on many islands, even in the middle of the widest oceans; as a valuable tax commodity.I have found that when stones of irregular shape are caught between the roots of trees, there are often small clumps of earth hidden in the gaps and behind the stones--they are so perfectly well-contained that they cannot be found during extremely long transports. A little was washed out; between the roots of an oak tree about 50 years old, there was a small patch of soil all tightly hidden, and on this small patch of soil three dicotyledonous plants sprouted: I know this observation to be true. of.I may also point out that the dead birds, floating at sea, are sometimes not eaten immediately, and that in the crops of such drifting dead birds there are seeds of many kinds, which remain alive for a long time: peas and vetch, for example. It takes only a few days to immerse in seawater to die: but the seeds in the crop of a pigeon that floated for 30 days in artificial seawater surprised me with almost all the seeds that germinated. Live birds are highly effective mediators in transporting seeds.I could give many facts to show how often birds of many species are carried over great distances by high winds.We may safely assume that, under such circumstances, the speed of flight may often have been 35 miles an hour; some authors have made higher estimates.I have never seen a case where the nutritious seeds passed through the guts of birds; but the hard seeds of the fruit can pass even through the digestive organs of a turkey without damage.Over the course of two months I have detected 12 species of seeds from bird droppings in my garden and they all seem to be in good condition, I have tried some seeds and they are still germinating.But the following fact is more important: that the crops of birds do not secrete gastric juices, and, according to my experiments, do not at all impair the germination of the seeds; We can safely assert that all the grains will not go into the sandbag within 12 or even 18 hours.A bird may easily be blown by the wind about 500 miles away during this period, and we know that hawks seek out weary birds, whose contents of torn crops may be thus easily scattered.Some hawks and owls (owls) swallow their prey whole, and over a period of twelve to twenty hours, in the clumps they spit out, I know from experiments done in zoos, still can sprout of seeds.Some seeds of oats, wheat, millet, canary, hemp, clover, and sugar beets germinate after twelve to twenty-one hours in the stomachs of various birds of prey; After two days and fourteen hours, the seeds still grew.I have found that freshwater fish eat the seeds of many types of terrestrial and aquatic plants, and that the fish are often eaten by birds, so that the seeds may be transported from one place to another.I have stuffed seeds of many species into the stomachs of dead fish, and then fed them to ospreys, storks, and pelicans, and after many hours the birds collected the seeds in small The clumps are spit out, or excreted in dung; and among these expelled seeds some retain their germinability.However, some seeds die after this process. Migratory locusts are sometimes carried by the wind to great distances from the land; I have caught one myself 370 miles off the coast of Africa, and I hear of others having caught them still further.The Rev. R. Lowe told Sir Lyle that in November 1844 a large swarm of migratory locusts had come to Madeira. They were innumerable, like snowflakes in a blizzard, reaching as high as just under a telescope. where you can see.During the second or third day, they flew rapidly in groups, slowly forming a large oval shape with a diameter of at least five or six miles, and landed on taller trees at night, and the trees were completely covered by them.They then disappeared over the sea as if they had suddenly appeared, and have not been there since.Some farmers in some parts of Natal now believe, though not well proved, that some noxious seeds are left on their meadows in the dung of the swarms of migratory locusts which frequent there.In this belief Mr. Weale once sent me in a letter a small packet of dried dung, which I examined under a microscope contained several seeds which, when planted, produced seven thatch plants. , belonging to two species and two genera.A swarm of locusts, such as that which flew to Madeira, might, therefore, easily transport several species of vegetation to islands far from the mainland. The beak and feet of birds, though generally clean, were sometimes soiled: I once removed sixty-one inches of dry clay from the foot of a partridge, and another twenty-two inches. centimeters, and in the soil there was a pebble the size of a vetch seed.And a better example: A friend sent me the leg of a woodcock, with a patch of dry earth stuck to the shin, weighing only nine inches, containing a grain of Juncus bufonius. Seeds can germinate and flower.Mr. Swaysland of Brighton, who has been observing our migratory birds closely for the last forty years, told me that he often takes Motacillae, Wheat-ears and European Stones [i.e. birds] (Saxicolae) first came to our shore, and knocked them down before they landed; several times he noticed small clumps of earth clinging to their feet.There are many facts which show that it is very common for soil to contain seeds.For example, Professor Newton (Prof. Newton) gave me the leg of a red-footed chutney (Caccabis rufa) that was injured and could not take off. It had a lump of dirt on it, weighing six and a half ounces.这块泥土被保存了三年，但是把它打碎后，浸湿，放在钟形玻璃罩下，不下82株植物从其中生长出来了：在这等植物里有12株单子叶植物，包含普通的燕麦和至少一种茅草在内，并且还有70株双子叶植物，从这些双子叶植物的幼叶来判断，至少有三个不同的物种。有这样的事实摆在我们面前，可知许多鸟类每年被大风吹过海洋的巨大空间，每年迁徙——例如，几百万只三趾鹑（quail）飞过地中海，它们一定会偶然地把附着在脚或喙上的污物中的种籽输送出去，对此我们还能有所怀疑吗？但是这个问题以后我还要讨论。 我们知道冰山有时载荷着土和石，甚至挟带着树枝、骨头和陆栖鸟类的巢，所以不必怀疑，如莱尔所提出的，它们一定有时在北极区和南极区把种籽从一处地方输送到另一处地方；而且在冰期，从现在的温带的一处地方把种籽输送到另一处地方。在亚速尔群岛上，如果拿靠近大陆的大西洋的其他岛屿上的物种来比较，它有更多和欧洲共通的植物，并且拿纬度来比较，这些植物多少带有北方的特性（如沃森先生所说的），我从这情形推测，这等岛屿上的种籽是在冰期部分地由冰带去的。我曾请求莱尔爵士写信给哈通先生（Mr.Hartung），问他在那些岛上是否看到过漂石，他回答说，他曾看到过花岗岩和其他岩石的巨大碎块，而这些岩石不是该群岛原来就有的。因此我们可以稳妥地推论，冰山从前曾把装来的岩石卸在这等海中央的群岛的岸上，这些岩石至少可能带来了少数北方植物的种籽。 考虑到这几种输送方法以及今后无疑将被发见的其他输送方法，几多万年以来，年复一年地起着作用，我想，许多植物如果没有这样被广泛输送出去，简直是奇怪的事情。这等输送方法有时被称为偶然的，但这不是严格正确的说法；海流不是偶然的，定期风的方向何尝是偶然的。这里应当注意，任何输送的方法很少能把种籽运到很远的距离：因为种籽如受海水作用的时间太久，就不能再保持它们的生活力；并且它们也不能在鸟类的嗉囊或肠子里长久携带。然而这等方法却足以通过几百英里宽的海面、或者从这岛到那岛、或者从大陆到邻近的岛进行偶然的输送，但不能从一个相距很远的大陆输送到另一个大陆。相距很远的大陆上植物区系不会因这等方法而混淆起来：它们仍然像今日一样，保持着区别分明。海流，由于它们的走向，不会把种籽从北美洲带到不列颠，虽然它们大概会而且实际把种籽从西印度带到我国的西部海岸，在那里，如果它们没有由于长久的海水浸泡而死去，大概也不会忍耐我们的气候的。差不多每年总有一两只陆鸟被风吹过整个大西洋，从北美洲来到爱尔兰和英格兰的西部海岸；但是只有一种方法可以使这等稀有的漂泊者来输送种籽，即用附着在它们的脚上或喙上的污物的方法，而这事情本身却是罕见的偶然之事。甚至在这种情形下，一粒种籽落在适宜的土壤上而达到成熟，其机会是何等之少啊！但是，因为像大不列颠那样生物繁多的岛，根据所能知道的，在最近的几世纪内没有通过偶然的输送方法从欧洲或者其他任何大陆容纳过移住者（很难证明这一点），从而就主张生物贫乏的岛，离大陆更远，便不会用相似的方法容纳移住者，如果这样想，就要犯重大的错误。如果有一百个种类的种籽或动物输入到一个岛，纵使这个岛的生物远不如不列颠的那样繁多，而能很好适应它的新家乡和归化的，大概不会多于一个种类。但在悠久的地质时期内，当那个岛正在隆起并且在那里没有繁多的生物栖息以前，对于偶然的输送方法的效果并不能作出有力的反对议论，在一个几乎是不毛的岛上，只有少数或者没有破坏性的昆虫或鸟类生存在那里，差不多每一粒偶然来到的种籽，如果有适宜的气候，大概都会发芽和成活的。 在冰期中的散布 在被数百英里低地隔开的山顶上有许多相同的植物和动物，而高山种是不能在低地上生活的，这是既知的关于同一物种生活在相距很远的地点而彼此间显然没有可能从一处地方迁徙到另一处地方的最动人事例之一。在阿尔卑斯（Alps）或比利牛斯（Pyrenees）的积雪区，和欧洲极北部分，有何等多的同种植物存在，这的确是值得注意的事实；但美国怀特山（White Mountains）上的植物和拉布拉多（Labrador）的植物完全相同，阿萨·格雷说，它们和欧洲最高山上的植物也几乎完全相同，这是更值得注意的。甚至早在1747年以前，这样的事实就使葛美伦（Gmelin）断言同一物种一定是在许多相距很远的地点被独立创造的；要不是阿加西斯和其他人士唤起了对于冰期的生物注意，我们也许要停留在这种信念里的。冰期，如我们以后就要讲到的，可给这等事实提供一个简单的解释，我们几乎有各种可以想像到的有机的和无机的证据来证明，在很近的地质时期内，欧洲中央部分和北美洲都是处于北极的气候之下的。苏格兰和威尔士的山岳用它们山腰的划痕、表面的磨光和带去的漂石，表明那里的山谷以前曾经充满了冰川，这比火后的房屋废墟更能清楚地说明以往的情形。欧洲气候的变化如此之大，以致在意大利北部古代冰川所留下的巨大冰碛上，现在已经长满了葡萄和玉蜀黍。在美国的大部分地方所看到的漂石和有划痕的岩石，明白地显示出从前那里有一个寒冷的时期。 从前冰期气候对于欧洲生物分布的影响，如福布斯所解释的，大致如下。但我们如果假定新冰期是慢慢来的，随后就像从前所发生的情形那样又慢慢的过去的，将会更容易地追踪这等变化。当寒冷到来，并且各个南方地带变得适于北方生物的时候，北方生物便会占据温带生物的从前地位。同时南方生物便会一步一步地南移，除非它们被障碍所阻挡，它们就要死亡。山上将会遮盖了雪和冰，从前的高山生物大概要降到平地来的。寒冷达到极点时，北极的动物群和植物群，便会布满欧洲的中央各地，向南一直可到阿尔卑斯和比利牛斯，甚至可以伸延到西班牙。现在美国的温带地区同样也会布满北极的植物和动物，而且它们和欧洲的那些植物和动物大概大致相同；因为我们假定曾向南方各地迁徙的现在北极圈的生物，在全世界都是显著一致的。 当温暖回转时，北极生物大概要向北退去，后面紧紧跟着的是更温和地区的生物。当山脚下的雪融解时，北极生物遂占据了这个清洁的融解的地方，温暖渐渐增加，雪渐渐向上方融解，它们也渐渐迁移到山上去，这时候它们的一部分兄弟们则启程北去。因此，到了温暖完全回转时，曾经共同生活在欧洲和北美洲低地的同种生物，又将再次见于“旧世界”和“新世界”的寒冷地区，以及相距很远的许多孤立的山顶上了。 这样，我们就能理解在非常远隔的各地，如北美和欧洲的高山，为什么许多植物是相同的。这样，我们还能理解为什么各个山脉的高山植物与其正北方或近乎正北方的北极类型更是特别地有关系：因为寒冷到来时的第一次迁徙以及温暖回转时的再迁徙，一般是向着正南和正北的。例如，苏格兰的高山植物，如沃森先生所说的，以及比利牛斯的高山植物，如雷蒙德（Ramond）所说的，更是和斯堪的纳维亚北部的植物特别地相似；美国的和拉布拉多的相似；西伯利亚山上的和俄国北极区的相似。因为这等观点是以从前确有的冰期为根据的，所以在我看来，它能极其满意地解释欧洲和美洲的高山植物以及寒带植物的现在分布状况；因此，当我们在其他地区发见同一物种生活在相距很远的山顶上，纵使没有其他证据，我们几乎也可以断定，较冷的气候从前曾经允许它们通过中间低地进行迁徙，而现在这个中间低地已变得太暖，不适于它们生存了。 因为北极类型随着气候的变化，起初向南方移动，后来再退回北方，所以它们在长途迁徙时，不会遇到任何重大不同的气候；并且因为它们是集体迁徒的，所以它们的相互关系不会受到很大的扰乱。因此，按照本书所恳切说明的原理，这等类型将不会发生很大的变异。但高山生物当温暖回转的时候就被隔离了，起初在山脚下，最终在山顶上，其情形就有些不同了；因为所有相同的北极物种都留在彼此相距很远的山脉中，而且能在那里生存，是不可能的事情；它们还很可能和古代高山物种相混合，这些古代高山物种在冰期开始以前一定已经生长在山上，并且在最冷的时期一定会暂时地被驱逐到平地上来；它们还会受到多少不同的气候的影响。它们的相互关系在某种程度上会因此受到扰乱，结果它们就容易发生变异；而且它们确曾发生了变异；如果我们拿欧洲几个大山脉上的高山植物和动物来互相比较，虽然许多物种还是相同的，有些却成为变种，有些成为可疑的类型或亚种，更有一些成为代表各个山脉的密切近似的但不相同的物种了。 在上述例证里，我曾假定这想像的冰期开始时，环绕北极地方的北极生物就像它们今日那样地一致。但是还必须假定，许多当时全世界亚北极的和某些少数温带的类型，也是相同的，因为今日生存在北美洲和欧洲的平原上以及低坡上的某些物种也是相同的；可以质问：我怎样解释在真的冰期开始时全世界的亚北极类型和温带类型一致的程度。今日“旧世界”和“新世界”的亚北极带以及北温带的生物是被整个大西洋和北太平洋隔开了。冰期中，“旧世界”和“新世界”的生物居住在比现在的位置更向南，它们一定更加完全地被更加广阔的海洋隔开了；所以很可以质问：同一物种在当时或者以前怎么能够进入这两个大陆。我相信它的解释在冰期开始前的气候性质。在新上新世时期，世界上大多数生物在种别上和今日是相同的，并且我有可靠的理由相信当时的气候要比今日暖和些。因此，我们可以假定，今日生活在纬度六十度之下的生物，在上新世的期间却生活在纬度六十六到六十六度之间的北极圈下的更北方；而现在的北极生物当时则生活在还要接近北极的中断陆地上。现在我们看一看地球仪，就可知道在北极圈下，有差不多连续的陆地从欧洲西部通过西伯利亚一直到美洲东部。这种环极陆地的连续性，使生物在较适宜的气候下可以自由迁徒，于是“旧世界”和“新世界”的亚北极生物和温带生物在冰期以前的假定一致性，便可得到解释。 根据上面所讲的各种理由，可以相信我们的大陆虽然经过地面水平的巨大变动，但长久保持了几乎相同的相对位置，我极愿意引伸上述观点，并作出如下推论，即在更早的和最热的时期，例如旧上新世的时期，大多数同样的植物和动物都是栖息在几乎连续的环极陆地上的；而且，无论“旧世界”或“新世界”的这等植物和动物，在冰期还没有开始的很久以前，随着气候的逐渐变冷，开始慢慢地南移。如我所相信的，我们在欧洲中部和美国可以看到大多数它们的后代已发生了变化。根据这种观点，我们就能理解为什么北美洲和欧洲的生物之间的关系很少是相同的，——如果考虑到两个大陆的距离以及它们被整个大西洋所隔开，就可以知道这是一个高度值得注意的关系。我们还能进一步理解某些观察者所提出的一件奇异的事实：第三纪末期的欧洲和美洲的生物之间的相互关系比起今日更为密切；因为在这等比较温暖的时期，“旧世界”和“新世界”的北部差不多被陆地连接在一起，可以作为一个桥梁供两处生物的迁徒，后来由于寒冷，这个桥梁就不能通行了。 在上新世的温度馒慢降低的期间，栖息在“新世界”和“旧世界”的共同物种即向北极圈以南迁徙，此后它们相互之间就要完全隔绝。就更温暖地方的生物来说，一定在很久的时期以前就发生了这种隔离。当这种植物和动物向南迁移的时候，就会在一处大地区与美洲土著生物相混合，而且势必和它们相竞争；在另一处大地区则和“旧世界”的生物相混合，而且也势必和它们相竞争。如果，各种事情都有利于它们发生大量变异——远比高山植物发生的变异为大，因为高山植物仅在极其近代的期间内被隔离在欧洲和北美洲的若干山脉上和北极陆地上。因此，当我们比较“新世界”和“旧世界”的温带地区的现存生物时，我们只找到很少数相同的物种（虽然阿萨·格雷最近指出两地植物相同的情况比从前料想的为多），但我们在每一个大纲里可以找到许多类型，某些博物学者把它们列为地理族，另外一些博物学者则把它们列为不同的物种；还有大量密切近似的或代表的类型被一切博物学者列为不同的物种。 陆地上如此，海里也是这样，海栖动物群在上新世、甚至在更早的期间沿着北极圈的连续岸边几乎一致地向南迁徙，根据变异的学说，便可解释今日完全隔离的海洋里生活的类型何以密切近似。这样，我想我们便能理解在温暖的北美洲东西两岸的至今仍然生存的和已经绝灭的类型之间的关系何以密切近似；我们还能理解更值得注意的一个事实，即栖息在地中海和日本海的许多甲壳类（如代那的可称赞的著作所描述的）、某种鱼类以及其他海栖动物的密切近似关系，——地中海和日本海今日已被整个的大陆和海洋的广大空间所隔开了。 现在或者先前栖息在北美洲东西两岸沿海的、地中海和日本海的以及北美洲和欧洲的温带陆地的物种之间的密切关系，是不能用创造学说来解释的。我们不能说，该地的物理条件是相似的，因而创造出来的物种也是相似的；因为，比方我们把南美洲的某些部分和南非洲或澳洲的某些部分加以比较，我们便知道这些地方的一切物理条件都是密切相似的，但它们的生物却完全不相似。 北方和南方的冰期交替 我们必须回到更直接的问题。我相信福布斯的观点大可扩展。在欧洲，从不列颠西海岸到乌拉尔（Oural）山脉，并且南到比利牛斯山，我们看到冰期的最明显的证据。根据冰冻的哺乳动物和山岳植被的性质，我们可以推论西伯利亚也曾受过相似的影响。胡克博士说，在黎巴嫩（Lebanon），从前有常期的积雪盖满了中脊，并且从此处出发的冰川下泻到四千英尺的山谷里。这位观察者最近在非洲北部的阿特拉斯（Atlas）山脉低处发见了大冰碛。沿着喜马拉雅山，在距离那里九英里的各地，冰川留下了它们从前下泻的痕迹；胡克博士在锡金（Sikkim）看到过玉蜀黍生长在古代的巨大的冰碛上。亚洲大陆南的赤道那一边，根据哈斯特博士（Dr．J. Haast）和海克托博士（Dr.Hector）的优秀研究，我们知道在新西兰从前曾有过巨大的冰川流到低地；胡克博士在这个岛上的隔离很远的山上发见有同样的植物，也说明了在那里从前曾经有过一段寒冷时期。根据克拉克牧师（Rev.WBClarke）写信告诉我的事实，澳洲东南角的山上显然也有从前冰川活动的痕迹。 看一看美洲；在它的北半部大陆的东侧，南至纬度36-37度处，曾发现由冰川带来的岩石碎片，在气候已经发生了很大变化的太平洋沿岸，南至纬度46度的地方也有这样发见。在落基山（Rocky Motuntains）上也曾看到过漂石。在近赤道之下的南美科迪勒拉山，冰川曾经一度远远扩张到它们今日的高度以下。我在智利的中部调查过一个含有大漂石的巨大岩屑堆，横穿泡地罗（Portillo）山谷，在那里无疑曾经一度形成过巨大的冰碛，而且福布斯先生告诉我说，他在南纬13到30度之间的科迪勒拉山的高约一万二千英尺的各地，发见了一些沟痕很深的岩石以及含有凹槽的小砾石的大岩屑堆，这些与他在挪威所习见者相类似。在科迪勒拉的这整个的区域内、甚至极高之处，今日也没有真正的冰川存在了。在这个大陆两边的更南方，从南纬41度到最南端，有无数的漂石都是从遥远的原产地运来的，在这里我们可以找到从前冰川活动的最明显的证据。 由于冰川的活动曾经扩展到南北两半球的全部——由于南北两半球的冰期按照地质学的意义来说都是属于近代的——由于南北两半球的冰期极其长久的持续，这是可以由它所发生的影响量推论出来的——最后，由于冰川最近曾沿科迪勒拉山全线下降至地平线——由于这几种事实，我在以前一个时期曾以为我们不可避免地要做出如下结论，即全世界的温度，在冰期曾经同时降低。但目前克罗尔先生在一系列可称赞的文章里曾企图说明，气候的结冰状态是各种物理原因的结果，而这等原因是由于地球轨道的离心性的增大才发生作用。所有这些原因都会导致同样的结果；但是，最有力的，似乎是轨道的离心性作用对于海流的间接影响。据克罗尔先生说，每一万年或一万五千年，寒冷时期会规则地循环；每隔长久的间歇时期，寒冷因为某些偶发事件，是极端严酷的；偶然事件中最重要的，如莱尔爵士所指出的，是水陆的相对位置。克罗尔先生相信最近的一次大冰期是在二十四万年以前，并且持续了约六万年，其间气候仅有微小的变化。关于更古的冰期，某些地质学者根据直接的证据，相信它们曾经出现在中新世和始新世的地质层，至于更古的地质层就不必提了。但是克罗尔先生得到的结果，对于我们最重要的是，当北半球经过寒冷时期的时候，南半球的温度主要由于海流方向的改变，实际上是升高了，它的冬季气候是很暖和的。反之，当南半球经过冰期的时候，北半球也是如此。这一结论非常有助于说明地理分布问题，所以我坚决地倾向于相信它；但我首先举出一些需要解释的事实。 在南美洲，胡克博士曾阐明，火地的显花植物（它们在该地贫乏的植物群中构成了不小的部分）除去许多密切近似的物种之外，有四十到五十种和相距辽远的、且处于另一半球内的北美洲和欧洲的植物相同。在赤道下的美洲高山上，生有大群属于欧洲属的特殊物种。在巴西的阿更山（Organ Mountains）山上，加得纳（Gardner）看到少数温带欧洲的属，一些南极的属，以及一些安第斯山（Andean）的属，这些属并不生于低下的中间热带地方。在加拉加斯（Caraccas）的西拉（Silla），著名的洪堡很久以前就发见了属于科迪勒拉山的特有属的物种。 在非洲的阿比西尼亚的山上，有若干欧洲的特有物种以及好望角的植物群的少数代表。在好望角，有很少数的欧洲物种可以相信不是人为引进去的，并且在山上有若干不见于非洲热带地方的若干欧洲代表类型。胡克博士近年也曾阐明，几内亚湾（Gulf of Guinea）内极高的费尔安多波（Fernando Po）岛的高地上以及邻近的喀麦隆山（Cameroon Mountains）上的若干植物，与阿比西尼亚山上的以及温带欧洲的植物之间的关系是密切的。我听胡克博士说，洛牧师在佛德角群岛（Cape Verde）上发见了这些温带植物。同样的温带类型差不多在赤道之下横穿非洲的整个大陆，一直扩张到威德角群岛的山上。自有植物分布记载以来，这是最惊人的事实之一。 在喜马拉雅山和印度半岛的与外界隔离的山脉上，在锡兰的高地上，以及在爪哇的火山顶上，生有完全相同的、或彼此代表的、并且同时代表欧洲的、但不见于中间炎热低地的许多植物。在爪哇的高峰上所采集的各属植物的目录，竟是欧洲小丘上采集物的一幅图画！还有更动人的事实，是生在婆罗洲山顶上的某些植物竟代表特殊的澳洲类型。某些这等澳洲类型，我听胡克博士说，沿着马六甲高地扩张出去，一面稀疏地散布在印度，一面向北去，直到日本。 在澳洲南方的山上，米勒博士曾发见过若干欧洲的物种；不是人为引进去的其他物种则生长在低地；胡克博士告诉我，见于澳洲的但不见于中间炎热地方的欧洲植物属可以被列成一个长的目录。在胡克博士的那部可称赞的《新西兰植区系概论》里，关于该大岛的某些植物也举出了类似的和动人的事实。因此，我们知道某些生长在世界各地热带的较高的高山上的植物，以及生长在南北温带平原上的植物，不是同一物种，就是同一物种的变种。然而必须注意，这等植物不是严格的北极类型；因为照沃森先生说，“从北极退向赤道，高山植物群或山岳植物群实际上逐步减少了北极的性质”。除却这等同一的和密切近似的类型外，还有许多生长在同样远隔地域的物种属于现在中间热带低地所没有的属。 这些简单的叙述只适用于植物；但是在陆栖动物方面，也可举出少数类似的事实。海栖动物中也有同样的情形；我愿援引最高权威代拿教授的一段叙述作为例子，他说“新西兰和大不列颠正处在地球上正相反对的位置，但是这两处地的甲壳类的密切相似，甚于世界的其他任何部分，这的确是一件可惊的事实”。理查森爵士也说，在新西兰，塔斯马尼亚（Tasmania）等海岸，有北方的鱼重现。胡克博士告诉我说，新西兰和欧洲有二十五个藻类的物种是共通的，但它们不曾见于中间的热带海中。 根据上述事实——即在横穿整个赤道非洲的高地上，沿着印度半岛直到锡兰和马来群岛，以及在并不如此显著地横穿热带南美洲的广大地面上，都有温带类型的存在，差不多可以确定：在从前的某一时期，无疑是在冰期的最严酷的期间，曾有相当数量的温带类型借住在这等大陆的赤道区域的各处低地。在这一时期，在海平面上的赤道地带气候大概和现在同纬度的五千英尺到六千英尺高处的气候差不多相同，甚至还要冷些。在最冷的时期，赤道区域的低地一定遮盖混生的热带植被和温带植被，就像胡克所描述的繁生在喜马拉雅山高四千英尺到五千英尺的低坡上的植物一样，不过温带类型大概占有较大的优势。还有，曼先生（Mr．Mann）在几内亚湾中的费尔安多波的多山岛上，发见了温带欧洲的类型开始出现在约五千英尺的高处。在巴拿马的山上，西曼博士（Dr. Seemann）只在二千英尺的高处发见了和墨西哥植被一样的植被，他说，“热带的类型和谐地与温带类型相混合”。 现在让我们看一看克罗尔先生的结论——当北半球遭遇到大冰期的极端寒冷的时候，南半球实际上要比平时暖和些；这一结论是否对于今日显然不能解释的两半球的温带地方和热带山岳上的各种生物的分布，给予任何明白的解释。冰期，如用年代来计算，必然是极长久的；我们如果记得某些归化的植物和动物在数世纪内曾经分布到何等广大的空间，那么，这一时期对于任何数量的迁徙将是绰绰有余的。当寒冷渐渐增强的时候，我们知道北极类型便侵入了温带地方；并且从刚才所举出的事实看来，某些较强壮的、优势的、分布最广的温带类型无疑会侵入赤道地带的低地。这等炎热的低地生物同时会移往南方的热带和亚热带地区，因为南半球在这个时期是比较温热的。当冰期将要完结的时候，因为两半球渐渐恢复了从前的温度，所以生活在赤道下的低地的北温带类型遂被驱逐到从前的家乡，或者被毁灭，而由从南方回来的赤道地带类型所代替。然而，有些北温带类型几乎肯定会登上任何邻近的高地，假如这地方有足够的高度，它们就会像欧洲山岳上的北极类型那样地长久生存在那里。甚至气候不完全适合于它们，它们也会生存的，因为温度的变化一定是很缓慢的，而植物又确有驯化的一定能力，它们把抵抗寒暑的不同的体力传递给后代的事情说明了这一点。 按照事情的正规进行，当轮到南半球蒙受严酷的冰期时，北半球大概要变得温暖些，于是南方的温带类型便会侵入赤道地带的低地。以前留在山上的北方类型现在就要走下山来而与南方类型混合在一起。南方类型到温暖回转时，仍然要回到从前的家乡，留下少数的物种在山上，并且携带着某些曾经从山上险要处走下来的北温带类型，一起走向南方。这样，我们就会在南北温带以及在中间热带地区高山上看到少数完全相同的物种。但是在这等山上或者相反半球上长久留下来的物种，是必须与许多新类型相竞争，并且会暴露在多少不同的物理条件之下；因此它们就会显著地易于变化，并在今日一般都作为变种或代表种而存在；实际的情形就是这样。我们还须记住，以前的冰期在两半球曾经几度出现；因为这就可以依据同样原理来解释许多十分不同的物种栖息在同样的远隔地域上，而且它们隶属于现今在中间炎热地带见不到的属。 关于美洲，胡克坚决主张，关于澳洲，得康多尔坚决主张，相同的或稍微变异了的物种从北向南的迁徙，多于从南向北的迁徙，这是一件值得注意的事实。然而，我们在婆罗洲和阿比西尼亚的山上还看到南方的类型。我猜想这种偏重于从北向南的迁徙，是由于北方陆地范围较大，并且由于北方类型在其故乡生存的数量较多，结果，通过自然选择和竞争，它们便较南方类型的完善化的阶段较高，即占有优势的力量。这样，在冰期的交替期间，当两群生物在赤道的地区相混合时，北方类型就较有力量，能够保持山上的位置，并且以后能和南方类型一同南移；但南方类型对于北方类型并不能做到这样。今日还有这种情形，我们看到很多的欧洲生物布满在拉普拉他、新西兰，并且较小程度地布满在澳洲，而且打败了那里的土著生物；然而，近二世纪或三世纪从拉普拉他，近四十年或五十年从澳洲，虽然有容易附着种籽的兽皮、羊毛以及其他媒介物大批地输入到欧洲，但是在北半球任何地方归化的南方类型却为数极少。不过印度的尼尔盖利山（Neilgherrie Mountains）提供了局部的例外；因为我听到胡克博士说，澳洲类型在那里迅速地繁殖了，而且归化了。在最后的大冰期以前，无疑地，热带山上一定充满了特有的高山类型；但是这等类型几乎到处都被在北方的较大地区和较完备的生物工厂中产生出来的更占优势的类型压倒了。在许多岛上，土著生物和外来的归化生物差不多数目相等，甚至已居少数；这是它们走向绝灭的第一阶段。山是陆地上的岛，山上的生物已屈服于在北方较大地域内产生出来的生物，这真像岛上生物已屈服于并继续屈服于由人力而归化的大陆生物。 同样的原理可以适用于北温带、南温带以及热带山上的陆栖动物和海栖生物的分布。在冰期的鼎盛期间，当海流和现在很不相同时，有些温带海洋的生物可能到达了赤道；其中的少数大概能乘着寒流立刻再向南迁徙，而其他则停留在和生存在较冷的深处中，直到南半球遇到冰期的气候时，它们才能更向前进；按照福布斯的意见，这种情形就和北极生物至今仍栖息于北方的温带海洋深处的孤立地方几乎是一样的。 我远非设想，今日生活在隔离得如此遥远的南方和北方、并且有时生活在中间山脉上的同一物种和近似物种的亲缘及其分布的所有难点，都可用上述观点来消除。我们还不能指出迁徙的精确路线。我们不能说明为什么某些物种迁徙了，而其他物种没有迁徙;为什么某些物种变异了并且产生了新类型，而其他物种却依然保持不变。直到我们能说明，为什么某一物种能够借人力在异乡归化，而其他物种不能如此；为什么某一物种比其家乡的另一物种分布得远至二倍或三倍，而且多至二倍或三倍，否则我们就不能希望解释上述事实。还有各种特别的难点留待解决：例如，胡克博士所阐明的，在凯尔盖朗岛（Kerguelen Land）、新西兰和富其亚（Fuegia）这样辽远的地点，生长着同样的植物；但按照莱尔的意见：冰山大概对于这些植物的分布有关系。在南半球的这等地方以及其他远隔地方生存的物种，虽然是不同的，但却完全属于南方的属，这是一个更值得注意的情形。有些物种是如此地不同，以致我们不能设想，自从最近的冰期开始以来，有足够的时间可供它们迁徙和此后进行必要程度的变异。这种事实似乎指明了同属的不同物种是从一个共同的中心点向四面八方迁徙的；并且我以为在南半球和在北半球一样，在最近的冰期开始以前，曾有一个比较温暖的时期，那时候，现在被冰覆盖着的南极地方，支持了一个高度特殊而孤立的植物群。可以设想，在最近冰期内这个植物群没有被消灭之前；少数类型由于偶然的输送方法以及由于现今已沉没了的岛屿作为歇脚点的帮助，就已经在南半球的各处地方广阔地散布开了。这样，美洲的澳洲的和新西兰的南岸，大概会稍微沾染上这种生物的特殊类型。 莱尔爵士在一篇动人的文章里，用着和我几乎一样的说法来推论全世界气候的大转变对于地理分布的影响。并且我们现在又看到克罗尔先生的结论——一个半球上的连续冰期和对面半球上的温暖期是一致的——和物种缓慢变化的观点一起解释了相同的或相似的生物类型分布在地球各处的许多事实。生命的水流在一个时期，从北向南流，在另一个时期，则从南向北流，在两种情形下都曾流到赤道