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Show 114 DARWINISM CliAI'. amounts, there seems no reason to believe that the necessity for two or more coincident variations would prevent the required change from taking place. The Continued Existence of Low FornU) of Life. Since species are continually undergoing modific<ttions giving them some superiority over other species or cnahling them to occupy fresh places in nature, it may be asked- \ Vhy do any low forms continue to exist~ Why have they not long since been improved :wd developed into higher forms~ Tlw answer, probably, is, that these low forms occupy places in nature which cannot be filled by higher forms, and that they huve few or no competitor ; they therefore continue to exist. Thus, earthworms arc adapted to their mode of life better than they would be if more highly organised. So, in the ocean, tho minute foraminifera and infusoria, and the larger sponges and corals, occupy pluccs which more higl1ly developed creatures could not fill. They form, as it were, the base of the great structure of animal life, on which tho next higher forms rest; and though in tho course of ages they may undergo somo changes, and diversification of form and structure, in accordance with changed condition. , choir essential nature has probably remained the same from the very dawn of life on the earth. The low aquatic diatomaccre and confervro, together with the lowest fun gi and lichens, occupy a similar position in the vegetable kingdom, filli ng places in nature which would be left vacant if only l1ighly organised plants existed. There is, therefore, no moti vc power to destroy or seriously to modify them; and they have thus probably persisted, under slightly varying forms, through all geological time. Extinction of Lower Types among the Ilighm· A nimals. So soon, however, as we approach the higher and more fully developed groups, we see indicn tion s of the often repeated extinction of lower by higher forms. This is show11 by the great gaps that separate the mammali:1, birds, reptiles, and fishes from each other ; while the lowest forms of each are always few in number and confined to limited areas. Such v NATURAL SELECTION 115 are the lowest mammals- the echidna and ornithorhynchus of Australia· the lowest birds-the apteryx of New Zealand and the ~assowarics of the Now Guinea region; while the lowest fish-tho amphioxus or la.ncelot, is completely isolated, and has apparently survived only by its habit of bu:rowing in the sand. The great distinctness of the carmvora, ruminants, rodents, whales, bats, and other orders of mammalia ; of the accipitrc. , pigeons, and parrots, _among birds ; and of the beetles, bees, flies, u,nd moths, among msccts, all indicate u,n enormous amount of extinction among the comparatively low forms by which, on any theory of evolution, these higher and more specialised groups must have been preceded. Circ1trnstances favourable to the Origin of New Species by Nat ural Selection. We have already seen that, when there is no change in the physical or organic conditions of a country, the efl'ect of natural selection is to keep all the species inhabiti11g it in a state of perfect health and full clcvclopmcnt, and to preserve the balance that already exists between the different t,rr·onps of organisms. But, whenever the physical or organic cond itions change, to however small an extent, some corre p_ondin()' chancre will be produced in the flora and fauna, sm ·c, co~sidcrigg the severe struggle for existence and the complex relations of the various organisms, it i hardly possible that the chan()'e should not be benefi cial to some species ancl hurtful t~ others. The most common effect, therefore, ''ill be that some species will increase and others will diminish ; and in cases where a species was already small in numbers a further diminution might lead to extinction. This would afford room for the increase of other species, and thus a considerable readjustment of the proportions of the several species might take place. When, however, the change was of a more important character, directly affecting the cxiste~ce ?f many species so as to render it difficult for them to mamtam themselves without some considerable change in structure or habits, that change would, in some cases, he brought about by variation and natural selection, and thus new varieties or new species might be formed. \V c have to consider, then, which |
