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Page:Popular Science Monthly Volume 73.djvu/550

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THE POPULAR SCIENCE MONTHLY

ity, or certain substances in solution, supplied at the proper moments. The obvious suggestion is, that species are more readily modified than is commonly admitted; and that in particular they are likely to be so modified on the borders of the territory they occupy, where they continually impinge on unaccustomed environments.

To show that this is possible is a most important step; but we still have to enquire how far has it actually occurred? In the case of our flax, we have an excellent example of the production of a new form on the periphery of the old, permitting expansion through modification; but only one such derivative seems to have been produced. In other instances, as the experimentalists have shown, the apparent instances are illusory the supposed geographical segregates being merely examples of a single type variously modified by the direct action of the environment. The most striking evidence of this sort has been furnished by Beebe, who has produced in certain birds, by means of humidity, more difference that has been accepted as sufficient for the distinction of subspecies. Leaving out all such phenomena, we still have a great series of closely allied species, with undoubtedly inherited characters, presenting the same kinds of differences as have been observed to arise by mutation, sometimes apparently as the direct result of particular stimuli. What do these phenomena mean in the practical working out of evolutionary processes?

If we know in a general way the age of particular types and the extent of their migrations, we can begin to form an idea of their practical mutability. The vertebrate paleontologist finds evidence of remarkable changes within the Tertiary period, but even he has to admit that the course of evolution is not so rapid as it might seem; that new forms suddenly appearing must surely have migrated from other regions, where they doubtless underwent a slow process of development. Central Asia, we must now think, must have been the home of various groups, and will one day yield fossils of surpassing interest. Africa, once seeming so barren paleontologically, has of late begun to yield her treasures.

The student of fossil invertebrates finds the process of change to have been, in the majority of cases, extraordinarily slow; and the paleobotanist finds it slower still. It is not that plenty of specific forms were not produced, but the generic and higher types were so little susceptible to change. A wrong impression has been produced, even among the vertebrates, by the presence in different strata of remarkable extinct groups. No doubt two or three species of elephants walking about in our mountain-parks would give a strikingly different appearance to the Colorado landscape; but the time since this actually occurred is, geologically speaking, very inconsiderable, and does not represent any great step in the process of evolution. The more I study the