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Page:Popular Science Monthly Volume 73.djvu/551

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ASPECTS OF MODERN BIOLOGY

insects and plants of the Florissant Miocene, the more convinced I become that, speaking broadly, the extinct genera and higher groups are not the ancestors of any now living, but represent types which have failed, like the mammoth; while the real representatives of the modern biota show that there has been singularly little forward evolution in the course of perhaps a million years. Many of these are totally extinct in Colorado, but live elsewhere; thus the redwood differs little from that of California, while the wonderfully delicate and fragile Halter, belonging to a family no longer living in North America, is closely related to a living species of Persia.

Hence the experimental researches of De Vries and others, proving that mutation is a relatively common phenomenon among plants, prove perhaps too much. If change is so easy, why so little change, and that in the face of a radical change in temperature and moisture? It seems, indeed, that "elementary species" have always been produced in greater or less abundance, but by a sort of oscillation less related to the forward march of evolutionary activity than we might at first suppose. The ability to produce heritable segregates, especially in the face of adverse or strange conditions, is clearly of advantage, as giving new chances for spread or survival. Thus in the long run the tendency to break into "elementary species" would in many cases be favored by natural selection, without any necessity for each one of these, or even the majority, being directly related to a particular environment. There is no reason, apparently, why this should not continue for ages as an oscillation-process, a segregation in space rather than in time, producing thousands of species without overstepping the limits of the general group, or perhaps advancing at all in complexity. The molluscan genus Ostrea, the oysters, may be taken as an example of this; indeed, the modern oysters scarcely do justice to their Cretaceous ancestors. When it was generally held that species were created by divine fiat, it naturally appeared that he who should explain the origin of species might be given the rest without further charge. We are coming to see that there are diverse problems involved, and while the whole matter may well be locked up in the evolution of any single species, or indeed of any single cell, we begin to doubt whether we really possess the key.

Speaking philosophically, progressive or orthogenetic evolution—the existence of which no naturalist has any ground for doubting—must have a cause external to itself. All probability favors the idea that this did not operate once for all, but has continued in action throughout the ages. It may be found, perhaps, in the susceptibility of the hereditary mechanism to environmental influences of particular kinds, the nature of which remains for the present obscure. These reactions would fall under the operation of natural selection from the very