Page:Popular Science Monthly Volume 64.djvu/458

454 basis are combined, perhaps inextricably, but all attempts at assigning them to a single cause or property have failed.

Cross-fertilization is commonly misunderstood to be merely an accessory of reproduction, and a negative factor in evolution, because it is supposed to conduce to the permanence of the specific type by averaging away the new characters which arise as individual variations. There is the amplest experimental evidence that cross-breeding is necessary to maintain the quality and efficiency of the individual, but static theories require us to believe that evolutionary progress requires conditions unfavorable to the individuals of which species are composed, since under such conditions selection is most effective, and abrupt variations are most striking and numerous. The alternative kinetic theory holds that cross-fertilization, as the active agency of symbasis, is a positive and primary factor of evolution, coordinate with variation itself. Symbasis is, as it were, the multiplier of the evolutionary equation, because it compels the distribution and combination of individual variations into the resultant vital motion of the species. Evolution no longer appears as an abnormal or exceptional phenomenon, and it becomes clear that the conditions under which the species is most prosperous are also those which permit the most rapid evolutionary progress.

The first corollary of the law of symbasis is the prepotency of variations. The combination of variations not only permits the structure of the organism to be strengthened and rendered more efficient, but also gives prepotency, due to the opportunity of vital motion. Variant individuals being thus both vigorous and prepotent, it is easy to understand why diversity, and not uniformity, is the tendency of normally extensive species; changes are necessary and welcome, and the perpetuation of them does not require segregation. Numerous and well