« ПредыдущаяПродолжить »
Maori saying is, "As the white man's rat has driven away the native rat, as the European fly drives away our own, and the clover kills our fern; so will the Maoris disappear before the white man himself." Similarly, within these few years an American water-weed has taken possession of our English ponds and rivers, and to some extent supplanted native water-weeds. A small kind of red ant, having habits allied to those of tropical ants, has of late overrun many houses in London. Rats and cockroaches have taken to infesting ships, and when the ships visit India, the oriental cockroach clears them of its European rival. Again, we may compare large tracts of land in Australia, South Africa, and Western South America, between latitudes 25° and 35°, where all the conditions are extremely similar, and yet the three faunas and floras are as dissimilar as it is possible for them to be.
There is a disposition on the part of all animals and plants to invade the territory occupied by others, and to establish themselves if possible; and, as in the case of human tribes, there are permanent conquests, temporary occupations, and occasional raids. It is not for want of adaptedness to other regions, nor for want of a disposition to push into them, that the organisms of a district are found where they are and nowhere else; it is because there is some physical barrier interveningan impassable ocean or isthmus or mountain chain. No two marine faunas are more distinct, with hardly a fish, shell, or crab, in common, than those of the eastern and western shores of South and Central America ;1 yet these great faunas are separated only by the narrow isthmus of Panama. Nearly all the terrestrial productions of 1 I write before the results of the Hassler expedition are published.
the New and Old Worlds differ greatly, excepting in the northern parts, where the land almost joins, and where, under a slightly different climate, there may have been free migration for the northern temperate forms, as there now is for the strictly Arctic productions. Frogs, toads, and newts seem peculiarly suited for a residence in islands for frogs introduced into Madeira, the Azores, and Mauritius, have so multiplied as to become a nuisance; and yet these creatures are absent from oceanic islands in general.
"The character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species, and thus its absence is not incongruous with the hypothesis that each species was originally placed in the regions most favourable to it. But the absence of a species from regions that are favourable to it cannot be thus accounted for. Were plants and animals localised wholly with reference to the fitness of their constitutions to surrounding conditions, we might expect floras to be similar, and faunas to be similar where the conditions are similar; and we might expect dissimilarities among floras and faunas proportionate to the dissimilarities of their conditions. But we do not find such anticipations verified."
Lastly, many animals possess rudimentary organs which are apparently useless, and which are sometimes fully developed in allied species. The whalebone whale has horny plates in its mouth, and no teeth; but the young fœtal whale, before birth, possesses teeth which never come to anything. Other whales, however, have welldeveloped teeth in both jaws. The horse has only one finger in his fore foot, and only one toe in his hind foot, each developed into a hoof, but it possesses rudimentary
splint-like bones, which correspond with bones belonging to certain toes and fingers in man; and the rhinoceros, which is closely allied to the horse anatomically, has the extra toes well formed. The external ears are represented by mere vestiges in a Chinese breed of sheep, and in another breed the tail is reduced "to a little button, suffocated in a manner by fat." In tailless cats and dogs a stump is left; in polled Suffolk cattle rudiments of horns can often be felt at an early age; in certain breeds of fowls the comb and wattles are reduced to rudiments. If these aborted organs appeared only now and then in individuals, where the rest of the species had them well developed, we should not be surprised, but when they characterise entire species from generation to generation, and their full development is the rare exception, our curiosity is excited to know the
§ 2. Some Facts of Paleontology.
It was shown in the section on the Evolution of the Earth's Crust, that the sedimentary rocks contain organic remains, which belonged to creatures existing when the rocks were forming. It must now be added that most of these fossils are of species that no longer exist, and many of them of genera that have passed away. An investigation of these buried forms brings out a number of facts analogous to those which we have just passed in review, in connection with the still living world.
First, it appears, as the late Edward Forbes often insisted, that there is a striking parallelism between the distribution of life in time, and its distribution in space: the laws governing the succession of forms in past times being nearly the same with those governing at
SUCCESSION OF FAUNAS.
the present time the differences in different areas. we descend through the series of rocks, the forms of life change, as they change if we walk from one end of a continent to the other and pass on to new continents. The fauna of any great period of the earth's history is intermediate in general character between that which preceded and that which succeeded it. The names given to the sub-divisions of the tertiary strata were intended to indicate the proportions of existing shells to extinct species-Eocene, the dawn of recent forms; Miocene, a minor proportion of recent forms; Pliocene, a comparative preponderance of recent forms; the Pliocene being the uppermost of the three divisions.1 Below the Tertiaries we get scarcely any forms precisely like those now extant; and although we still find species allied to them, the unlikeness increases as we go downwards. Past faunas differ from each other as well as from the present, and the differences between them are proportionate to their degrees of remoteness from each other in time, as measured by their relative positions in the sedimentary series. So that if we take the assemblage of organic forms living now, and compare it with the successive assemblages of organic forms that have lived in successive geologic epochs, we find that the farther we go back into the past, the greater does the unlikeness become; the number of species and genera common to the compared assemblages becomes smaller and smaller, and the assemblages differ more and more in their general character. The divergence is comparatively slow and continuous where there is continuity in the geological formations; but is
1 nás, eōs, dawn, and xavós, kainos, recent; μɛíwv, meiōn, less; Thelwv, pleion, more.
sudden and comparatively wide wherever there occurs a great break in the succession of strata. Stratigraphical breaks are due either to non-deposition, or to subsequent removal of deposits, and must consequently be local only, though we may not always succeed in finding intermediate strata (with intermediate forms of life) elsewhere. But we can say confidently, that as there never was a time when there were no land surfaces, so there has never been a break in the process of sedimentation, and in the succession of life. Speaking generally, the endurance of each species and group of species is continuous in time as it is in space; both in time and space species, and groups of species, have their points of maximum development; groups belonging either to a certain time or a certain area are often characterised by trifling characters in common, as of sculpture or colour; in looking to the long succession of ages as in looking to distant provinces of the existing world, we find that some organisms differ little, whilst others belonging to a different class, or to a different order, or even only to a different family of the same order, differ greatly.
The second fact-a corollary of the first-is, that no group or species has come into existence twice. It cannot be said that the present conditions of the earth are unsuited to the existence of all the living things that have passed away, any more than it can be said that the waters which wash the Isthmus of Panama on the Pacific side are incapable of sustaining the life which exists on the Atlantic side; but species have found barriers in the flow of time, as they find them in the stretch of space.
A third fact is, that just as all existing species are connected together by relationships of form and struc