Page:Popular Science Monthly Volume 63.djvu/22

18 in any evolutionary sense. The wonder is not that organisms build differently with different materials, but that they are able to build with the same materials such infinite diversity of form and structure.

That with adequate materials supplied by abundant food a species would be able to exhibit a larger range of variation, is undoubtedly true, and offers no difficulties in a kinetic theory. The more favorable the conditions or the more successful the adaptation, the more numerous the individuals; also the more extensive would be the manifestations of the variational possibilities of the species, and the more rapid the resulting evolutionary progress. If static theories of evolution were correct numerical increase would not favor evolutionary change because it would diminish the chances of the segregation on which the preservation of variations has been thought to depend.

The most advanced organic types—those which have traveled farthest on the evolutionary journey—are not natives of islands, but of continents. The greatest and most rapid evolutionary progress has not been made among organisms of localized distribution, but among those having facilities for wide dissemination and free interbreeding. Large species move faster than small. Insular species become diverse from their continental relatives mainly because they are left behind by the latter rather than because isolation favors evolution.

Segregation did not denote evolution either in the remote or in the more recent past. As the geological record is followed backward the more generalized types are found to have more generalized distribution, and if in former ages evolutionary changes were more rapid than at present in any particular group this may well be correlated with a period of very favorable conditions permitting the simultaneous existence of vast numbers of individuals in species continuous over large areas. The later subdivision of these generalized types betokens less favorable circumstances which reduced the numbers or otherwise localized the distribution, and thus segregated the new groups. The birds outnumber the reptiles, the insects the myriapods, the composites the palms. The better the facilities for distribution the more rapid the evolution.

On the other hand the greater the localization and the fewer the individuals the slower the evolutionary progress of a species, and the more uniform the characters. Their supposed constancy leads