Page:Popular Science Monthly Volume 64.djvu/455

Rh and with any or all its characters changed. It is not necessary even that the individuals of a species remain alike; in many unrelated natural groups extremely diverse sexes, castes and 'forms' remain associated in the same species and travel together on the evolutionary journey, sharing the same environment, but without any tendency to become 'exactly alike.' Moreover, we know that sexual and other diversities inside the species are not casual or accidental, but normal and advantageous, facts quite overlooked in static theories, which have viewed life from a narrowly systematic standpoint and have argued that interbreeding prevents the preservation of new characters and is thus a hindrance to evolution.

The kinetic theory, on the contrary, ascribes the fact that organisms are everywhere bound up into species to a property of fundamental evolutionary importance, and interprets the multitudinous devices for maintaining the coherence of groups of interbreeding organic individuals and the equally general manifestations of sexual and other diversification inside specific lines, as due to the same requirement of protoplasmic organization, an interlacing network of descent. Without cross-fertilization species would not cohere, but would split into numberless independent, diverging lines. This takes place with organisms long propagated asexually, whether artificially or in nature. For example, the genus Sphagnum, which very rarely produces spores, offers a multiplicity of varieties nowhere approached among mosses having normal sexual reproduction; but notwithstanding so many differences in minute details Sphagnum has remained a very compact, unprogressive group. Cross-fertilization prevents this type of diversification, but it need not on that account be supposed to impede evolutionary progress. Evolution is not merely a progressive diversification, it requires also a progressive synthesis of characters by the interbreeding of the individual members of specific groups.

That sexual reproduction is a substitute or improvement of multiplication by fission is another partial and misleading view which has contributed much towards the concealment of the causes of evolution. The division of cells is the only method of organic increase; conjugation is not multiplication, but serves as a preliminary stimulant to the necessary cell-division. What is growth, for example, among the filamentous algæ composed of chains of cells is reproduction among the unicellular species, where divided cells become separate individuals.