At about the same period when icebergs were transporting boulders in N.
America as far as 36 south, where the cotton tree now grows in South America, in lat.i.tude 42 (where the land is now clothed with forests having an almost tropical aspect with the trees bearing epiphytes and intertwined with canes), the same ice action was going on; is it not then in some degree probable that at this period the whole tropical parts of the two Americas possessed{370} (as Falconer a.s.serts that India did) a more temperate climate? In this case the Alpine plants of the long chain of the Cordillera would have descended much lower and there would have been a broad high-road{371} connecting those parts of North and South America which were then frigid. As the present climate supervened, the plants occupying the districts which now are become in both hemispheres temperate and even semi-tropical must have been driven to the Arctic and Antarctic{372} regions; and only a few of the loftiest points of the Cordillera can have retained their former connecting flora. The transverse chain of Chiquitos might perhaps in a similar manner during the ice-action period have served as a connecting road (though a broken one) for Alpine plants to become dispersed from the Cordillera to the highlands of Brazil. It may be observed that some (though not strong) reasons can be a.s.signed for believing that at about this same period the two Americas were not so thoroughly divided as they now are by the West Indies and tableland of Mexico. I will only further remark that the present most singularly close similarity in the vegetation of the lowlands of Kerguelen"s Land{373} and of Tierra del Fuego (Hooker), though so far apart, may perhaps be explained by the dissemination of seeds during this same cold period, by means of icebergs, as before alluded to{374}.
{370} Opposite to this pa.s.sage, in the margin, the author has written:--"too hypothetical."
{371} The Cordillera is described as supplying a great line of invasion in the _Origin_, Ed. i. p. 378.
{372} This is an approximation to the author"s views on trans-tropical migration (_Origin_, Ed. i. pp. 376-8). See Thiselton-Dyer"s interesting discussion in _Darwin and Modern Science_, p. 304.
{373} See Hooker"s _Lecture on Insular Floras_ in the _Gardeners"
Chronicle_, Jan. 1867.
{374} Similarity of flora of coral islands easily explained.Finally, I think we may safely grant from the foregoing facts and reasoning that the anomalous similarity in the vegetation of certain very distant mountain-summits is not in truth opposed to the conclusion of the intimate relation subsisting between proximity in s.p.a.ce (in accordance with the means of transport in each cla.s.s) and the degree of affinity of the inhabitants of any two countries. In the case of several quite isolated mountains, we have seen that the general law holds good.
_Whether the same species has been created more than once._
As the fact of the same species of plants having been found on mountain-summits immensely remote has been one chief cause of the belief of some species having been contemporaneously produced or created at two different points{375}, I will here briefly discuss this subject. On the ordinary theory of creation, we can see no reason why on two similar mountain-summits two similar species may not have been created; but the opposite view, independently of its simplicity, has been generally received from the a.n.a.logy of the general distribution of all organisms, in which (as shown in this chapter) we almost always find that great and continuous barriers separate distinct series; and we are naturally led to suppose that the two series have been separately created. When taking a more limited view we see a river, with a quite similar country on both sides, with one side well stocked with a certain animal and on the other side not one (as is the case with the Bizcacha{376} on the opposite sides of the Plata), we are at once led to conclude that the Bizcacha was produced on some one point or area on the western side of the river. Considering our ignorance of the many strange chances of diffusion by birds (which occasionally wander to immense distances) and quadrupeds swallowing seeds and ova (as in the case of the flying water-beetle which disgorged the eggs of a fish), and of whirlwinds carrying seeds and animals into strong upper currents (as in the case of volcanic ashes and showers of hay, grain and fish{377}), and of the possibility of species having survived for short periods at intermediate spots and afterwards becoming extinct there{378}; and considering our knowledge of the great changes which _have_ taken place from subsidence and elevation in the surface of the earth, and of our ignorance of the greater changes which _may have_ taken place, we ought to be very slow in admitting the probability of double creations. In the case of plants on mountain-summits, I think I have shown how almost necessarily they would, under the past conditions of the northern hemisphere, be as similar as are the plants on the present Arctic sh.o.r.es; and this ought to teach us a lesson of caution.
{375} On centres of creation see _Origin_, Ed. i. p. 352, vi. p.
499.
{376} In the _Journal of Researches_, Ed. 1860, p. 124, the distribution of the Bizcacha is described as limited by the river Uruguay. The case is not I think given in the _Origin_.
{377} In the _Origin_, Ed. i. a special section (p. 356, vi. p.
504) is devoted to _Means of Dispersal_. The much greater prominence given to this subject in the _Origin_ is partly accounted for by the author"s experiments being of later date, _i.e._ 1855 (_Life and Letters_, vol. II. p. 53). The carriage of fish by whirlwinds is given in the _Origin_, Ed. i. p. 384, vi. p.
536.
{378} The case of islands serving as halting places is given in the _Origin_, Ed. i. p. 357, vi. p. 505. But here the evidence of this having occurred is supposed to be lost by the subsidence of the islands, not merely by the extinction of the species.
But the strongest argument against double creations may be drawn from considering the case of mammifers{379} in which, from their nature and from the size of their offspring, the means of distribution are more in view. There are no cases where the same species is found in _very remote_ localities, except where there is a continuous belt of land: the Arctic region perhaps offers the strongest exception, and here we know that animals are transported on icebergs{380}. The cases of lesser difficulty may all receive a more or less simple explanation; I will give only one instance; the nutria{381}, I believe, on the eastern coast of S. America live exclusively in fresh-water rivers, and I was much surprised how they could have got into rivulets, widely apart, on the coast of Patagonia; but on the opposite coast I found these quadrupeds living exclusively in the sea, and hence their migration along the Patagonian coast is not surprising. There is no case of the same mammifer being found on an island far from the coast, and on the mainland, as happens with plants{382}. On the idea of double creations it would be strange if the same species of several plants should have been created in Australia and Europe; and no one instance of the same species of mammifer having been created, or aboriginally existing, in two as nearly remote and equally isolated points. It is more philosophical, in such cases, as that of some plants being found in Australia and Europe, to admit that we are ignorant of the means of transport. I will allude only to one other case, namely, that of the Mydas{383}, an Alpine animal, found only on the distant peaks of the mountains of Java: who will pretend to deny that during the ice period of the northern and southern hemispheres, and when India is believed to have been colder, the climate might not have permitted this animal to haunt a lower country, and thus to have pa.s.sed along the ridges from summit to summit? Mr Lyell has further observed that, _as in s.p.a.ce, so in time_, there is no reason to believe that after the extinction of a species, the self-same form has ever reappeared{384}. I think, then, we may, notwithstanding the many cases of difficulty, conclude with some confidence that every species has been created or produced on a single point or area.
{379} "We find no inexplicable cases of the same mammal inhabiting distant points of the world." _Origin_, Ed. i. p. 352, vi. p. 500.
See also _Origin_, Ed. i. p. 393, vi. p. 547.
{380} Many authors. < p="">394, vi. p. 547.>
{381} _Nutria_ is the Spanish for otter, and is now a synonym for _Lutra_. The otter on the Atlantic coast is distinguished by minute differences from the Pacific species. Both forms are said to take to the sea. In fact the case presents no especial difficulties.
{382} In _Origin_, Ed. i. p. 394, vi. p. 548, bats are mentioned as an explicable exception to this statement.
{383} This reference is doubtless to _Mydaus_, a badger-like animal from the mountains of Java and Sumatra (Wallace, _Geographical Distribution_, ii. p. 199). The instance does not occur in the _Origin_ but the author remarks (_Origin_, Ed. i. p. 376, vi. p.
527) that cases, strictly a.n.a.logous to the distribution of plants, occur among terrestrial mammals.
{384} See _Origin_, Ed. i. p. 313, vi. p. 454.
_On the number of species, and of the cla.s.ses to which they belong in different regions._
The last fact in geographical distribution, which, as far as I can see, in any way concerns the origin of species, relates to the absolute number and nature of the organic beings inhabiting different tracts of land. Although every species is admirably adapted (but not necessarily better adapted than every other species, as we have seen in the great increase of introduced species) to the country and station it frequents; yet it has been shown that the entire difference between the species in distant countries cannot possibly be explained by the difference of the physical conditions of these countries. In the same manner, I believe, neither the number of the species, nor the nature of the great cla.s.ses to which they belong, can possibly in all cases be explained by the conditions of their country. New Zealand{385}, a linear island stretching over about 700 miles of lat.i.tude, with forests, marshes, plains and mountains reaching to the limits of eternal snow, has far more diversified habitats than an equal area at the Cape of Good Hope; and yet, I believe, at the Cape of Good Hope there are, of phanerogamic plants, from five to ten times the number of species as in all New Zealand. Why on the theory of absolute creations should this large and diversified island only have from 400 to 500 (? Dieffenbach) phanerogamic plants? and why should the Cape of Good Hope, characterised by the uniformity of its scenery, swarm with more species of plants than probably any other quarter of the world? Why on the ordinary theory should the Galapagos Islands abound with terrestrial reptiles? and why should many equal-sized islands in the Pacific be without a single one{386} or with only one or two species? Why should the great island of New Zealand be without one mammiferous quadruped except the mouse, and that was probably introduced with the aborigines? Why should not one island (it can be shown, I think, that the mammifers of Mauritius and St Iago have all been introduced) in the open ocean possess a mammiferous quadruped? Let it not be said that quadrupeds cannot live in islands, for we know that cattle, horses and pigs during a long period have run wild in the West Indian and Falkland Islands; pigs at St Helena; goats at Tahiti; a.s.ses in the Canary Islands; dogs in Cuba; cats at Ascension; rabbits at Madeira and the Falklands; monkeys at St Iago and the Mauritius; even elephants during a long time in one of the very small Sooloo Islands; and European mice on very many of the smallest islands far from the habitations of man{387}. Nor let it be a.s.sumed that quadrupeds are more slowly created and hence that the oceanic islands, which generally are of volcanic formation, are of too recent origin to possess them; for we know (Lyell) that new forms of quadrupeds succeed each other quicker than Mollusca or Reptilia. Nor let it be a.s.sumed (though such an a.s.sumption would be no explanation) that quadrupeds cannot be created on small islands; for islands not lying in mid-ocean do possess their peculiar quadrupeds; thus many of the smaller islands of the East Indian Archipelago possess quadrupeds; as does Fernando Po on the West Coast of Africa; as the Falkland Islands possess a peculiar wolf-like fox{388}; so do the Galapagos Islands a peculiar mouse of the S. American type. These two last are the most remarkable cases with which I am acquainted; inasmuch as the islands lie further from other land. It is possible that the Galapagos mouse may have been introduced in some ship from the S. American coast (though the species is at present unknown there), for the aboriginal species soon haunts the goods of man, as I noticed in the roof of a newly erected shed in a desert country south of the Plata. The Falkland Islands, though between 200 and 300 miles from the S. American coast, may in one sense be considered as intimately connected with it; for it is certain that formerly many icebergs loaded with boulders were stranded on its southern coast, and the old canoes which are occasionally now stranded, show that the currents still set from Tierra del Fuego. This fact, however, does not explain the presence of the _Canis antarcticus_ on the Falkland Islands, unless we suppose that it formerly lived on the mainland and became extinct there, whilst it survived on these islands, to which it was borne (as happens with its northern congener, the common wolf) on an iceberg, but this fact removes the anomaly of an island, in appearance effectually separated from other land, having its own species of quadruped, and makes the case like that of Java and Sumatra, each having their own rhinoceros.
{385} The comparison between New Zealand and the Cape is given in the _Origin_, Ed. i. p. 389, vi. p. 542.
{386} In a corresponding discussion in the _Origin_, Ed. i. p. 393, vi. p. 546, stress is laid on the distribution of Batrachians not of reptiles.
{387} The whole argument is given--more briefly than here--in the _Origin_, Ed. i. p. 394, vi. p. 547.
{388} See _Origin_, Ed i. p. 393, vi. p. 547. The discussion is much fuller in the present Essay.
Before summing up all the facts given in this section on the present condition of organic beings, and endeavouring to see how far they admit of explanation, it will be convenient to state all such facts in the past geographical distribution of extinct beings as seem anyway to concern the theory of descent.
SECTION SECOND.
_Geographical distribution of extinct organisms._
I have stated that if the land of the entire world be divided into (we will say) three sections, according to the amount of difference of the terrestrial mammifers inhabiting them, we shall have three unequal divisions of (1st) Australia and its dependent islands, (2nd) South America, (3rd) Europe, Asia and Africa. If we now look to the mammifers which inhabited these three divisions during the later Tertiary periods, we shall find them almost as distinct as at the present day, and intimately related in each division to the existing forms in that division{389}. This is wonderfully the case with the several fossil Marsupial genera in the caverns of New South Wales and even more wonderfully so in South America, where we have the same peculiar group of monkeys, of a guanaco-like animal, of many rodents, of the Marsupial Didelphys, of Armadilloes and other Edentata. This last family is at present very characteristic of S. America, and in a late Tertiary epoch it was even more so, as is shown by the numerous enormous animals of the Megatheroid family, some of which were protected by an osseous armour like that, but on a gigantic scale, of the recent Armadillo. Lastly, over Europe the remains of the several deer, oxen, bears, foxes, beavers, field-mice, show a relation to the present inhabitants of this region; and the contemporaneous remains of the elephant, rhinoceros, hippopotamus, hyaena, show a relation with the grand Africo-Asiatic division of the world. In Asia the fossil mammifers of the Himalaya (though mingled with forms long extinct in Europe) are equally related to the existing forms of the Africo-Asiatic division; but especially to those of India itself. As the gigantic and now extinct quadrupeds of Europe have naturally excited more attention than the other and smaller remains, the relation between the past and the present mammiferous inhabitants of Europe has not been sufficiently attended to. But in fact the mammifers of Europe are at present nearly as much Africo-Asiatic as they were formerly when Europe had its elephants and rhinoceroses, etc.; Europe neither now nor then possessed peculiar groups as does Australia and S. America. The extinction of certain peculiar forms in one quarter does not make the remaining mammifers of that quarter less related to its own great division of the world: though Tierra del Fuego possesses only a fox, three rodents, and the guanaco, no one (as these all belong to S. American types, but not to the most characteristic forms) would doubt for one minute
{389} See _Origin_, Ed. i. p. 339, vi. p. 485.
{390} In the _Origin_, Ed. i. p. 339, vi. p. 485, which corresponds to this part of the present Essay, the author does not make a separate section for such cases as the occurrence of fossil Marsupials in Europe (_Origin_, Ed. i. p. 340, vi. p. 486) as he does in the present Essay; see the section on _Changes in geographical distribution_, p. 177.
We find some evidence of the same general fact in a relation between the recent and the Tertiary sea-sh.e.l.ls, in the different main divisions of the marine world.
This general and most remarkable relation between the lately past and present mammiferous inhabitants of the three main divisions of the world is precisely the same kind of fact as the relation between the different species of the several sub-regions of any one of the main divisions. As we usually a.s.sociate great physical changes with the total extinction of one series of beings, and its succession by another series, this ident.i.ty of relation between the past and the present races of beings in the same quarters of the globe is more striking than the same relation between existing beings in different sub-regions: but in truth we have no reason for supposing that a change in the conditions has in any of these cases supervened, greater than that now existing between the temperate and tropical, or between the highlands and lowlands of the same main divisions, now tenanted by related beings. Finally, then, we clearly see that in each main division of the world the same relation holds good between its inhabitants in time as over s.p.a.ce{391}.
{391} "We can understand how it is that all the forms of life, ancient and recent, make together one grand system; for all are connected by generation." _Origin_, Ed. i. p. 344, vi. p. 491.
_Changes in geographical distribution._
If, however, we look closer, we shall find that even Australia, in possessing a terrestrial Pachyderm, was so far less distinct from the rest of the world than it now is; so was S. America in possessing the Mastodon, horse, [hyaena,]{392} and antelope. N. America, as I have remarked, is now, in its mammifers, in some respects neutral ground between S. America and the great Africo-Asiatic division; formerly, in possessing the horse, Mastodon and three Megatheroid animals, it was more nearly related to S. America; but in the horse and Mastodon, and likewise in having the elephant, oxen, sheep, and pigs, it was as much, if not more, related to the Africo-Asiatic division. Again, northern India was much more closely related (in having the giraffe, hippopotamus, and certain musk-deer) to southern Africa than it now is; for southern and eastern Africa deserve, if we divide the world into five parts, to make one division by itself. Turning to the dawn of the Tertiary period, we must, from our ignorance of other portions of the world, confine ourselves to Europe; and at that period, in the presence of Marsupials{393} and Edentata, we behold an _entire_ blending of those mammiferous forms which now eminently characterise Australia and S.
America{394}.
{392} The word _hyaena_ is erased. There appear to be no fossil Hyaenidae in S. America.
{393} See note 1{390}, p. 175, also _Origin_, Ed. i. p. 340, vi. p. 486.
{394} And see Eocene European mammals in N. America.If we now look at the distribution of sea-sh.e.l.ls, we find the same changes in distribution. The Red Sea and the Mediterranean were more nearly related in these sh.e.l.ls than they now are. In different parts of Europe, on the other hand, during the Miocene period, the sea-sh.e.l.ls seem to have been more different than at present. In{395} the Tertiary period, according to Lyell, the sh.e.l.ls of N. America and Europe were less related than at present, and during the Cretaceous still less like; whereas, during this same Cretaceous period, the sh.e.l.ls of India and Europe were more like than at present. But going further back to the Carbonaceous period, in N. America and Europe, the productions were much more like than they now are{396}. These facts harmonise with the conclusions drawn from the present distribution of organic beings, for we have seen, that from species being created in different points or areas, the formation of a barrier would cause or make two distinct geographical areas; and the destruction of a barrier would permit their diffusion{397}. And as long-continued geological changes must both destroy and make barriers, we might expect, the further we looked backwards, the more changed should we find the present distribution.
This conclusion is worthy of attention; because, finding in widely different parts of the same main division of the world, and in volcanic islands near them, groups of distinct, but related, species;--and finding that a singularly a.n.a.logous relation holds good with respect to the beings of past times, when none of the present species were living, a person might be tempted to believe in some mystical relation between certain areas of the world, and the production of certain organic forms; but we now see that such an a.s.sumption would have to be complicated by the admission that such a relation, though holding good for long revolutions of years, is not truly persistent.
{395} All this requires much verification.{396} This point seems to be less insisted on in the _Origin_.