Page:Popular Science Monthly Volume 33.djvu/260

248 environment slow enough to he accompanied by change of the species depending upon it. There can he but little doubt that many if not most or all of existing species are slowly changing in mass and in place into something else than what they are now. This process may be so slow that it will require some such careful study and delicate calculation as has been used by astronomers to prove that the so-called fixed stars are in motion; but the well-founded belief that geological changes are still going on upon the earth is enough to make us certain that changes ' in living things must accompany these or follow close after.

In addition to such slow changes of the mass of individuals of a species as must finally produce a form specifically different from it, there is another method of formation of species which would seem to be much more prolific in its results, and which alone can keep the earth populated during ages of changes of environment, some of them so rapid and violent that they destroy great numbers of species. This is by the migration of individuals into surrounding areas where the conditions are so near like those to which they are accustomed that they can exist, and still different enough to set up rapid changes of structure. The offshoots of the parent species might thus become numerous, and still retain likeness enough to each other so that they would be thrown by the systematist into a common genus; or, if the changes of environment were less, we should have a set of subspecies or varieties. There is no line or rule fixed in nature by which we can say that this is a genus with several species, and this is a species with several varieties. If this is a true theory for the creation of species, there should exist certain species settled and fixed in character, which have existed with but the slow modifications of structures caused by geological changes, while other species would be ready to change in any direction, or to revert rapidly to the characters of its parent species. The varieties and species of man's make are exaggerated examples of the latter class, and man himself would furnish illustrations of the same thing in such ancient and well-established species as the Chinese, and such a recently-formed variety as the Anglo-American. The heterogeneous character of the conditions of environment bearing upon life, and their utter lack of equality or equivalence of modifying power, give good and sufficient rear son for that lack of equality of structural values among groups, which is best shown by the inability of the best systematists to agree upon their value. The fate of the group called species is a case in point. As long as the animals of a single well-defined area were studied, there was no difficulty, for closely-allied species necessarily rarely or never inhabit the same area; but as soon as the study became comparative, by bringing in forms from neigh