Page:Popular Science Monthly Volume 68.djvu/501

Rh organisms, notably in man, in other mammals and in birds. They form a large factor in the development of the individual. The education and training of the individual man produces functional variations, as distinguished from innate peculiarities. But the groups characterized by functional variations are of the nature of 'ontogenetic species.' There is no evidence that the current of heredity is affected by changes of this kind, or that they have any direct effect in the formation of true species. At the most, use or disuse of organs seems to affect species only in an indirect way, as by the preservation of those most disposed to functional activity, and which by such activity have been able to meet better the demands of the environment.

The preservation of individual variations with their extension to posterity gives rise to racial changes, and these to the larger variations which mark change in species.

Collective variations are chiefly of geographical or geological origin: Changes with space produce geographical species and subspecies. To this category belongs the vast majority of species and subspecies recognized in systematic zoology and botany. It is illustrated by the species of wood warbler (Dendroica) mentioned in a previous paper.

Changes with time produce geological mutations. It is a fact unquestionable that a species will change on its own grounds little by little with the lapse of time and the slow alteration of conditions of selection. Nations change, languages change, customs change, nothing is secure against the tooth of time. This is in general true, because with time alteration of environment takes place, events happen, there is an alteration of the stress of life and with this alteration all life may be modified.

That time-mutations in all forms of life do take place is beyond question, and some have regarded these slow changes as the chief agency in the formation of species. But the current of life does not flow in straight lines nor in an even current. Species are torn apart by obstacles, as streams are divided by rocks, and the rapidity of their formation is proportioned to the size of the obstacle and the alternations it produces in the flow of life.

We have some basis for the estimate of the duration of a species. When the great glacial Lake Bonneville occupied the basin of the Great Salt Lake, the same species of fishes and insects were found in all its tributaries. Now that these streams flow separately into a lifeless lake, the same species of fishes occur in them for the most part without alteration. One species of sucker (Catostomus ardens) and one chub (Leuciscus lineatus) are found unaltered throughout this region and in the Upper Snake River (above Shoshone Falls), into