THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 407 



and have argued that interbreeding is a hindrance to evohition in 

 that it pivvents the preservation of new characters. 



The kinetic theory, on the contrary, ascribes the fact that organisms 

 are every^Yhere bound up into species to a property of funchiniental 

 evohitionary importance, and interprets the nndtitiidinous devices 

 for maintaining the coherence of groups of interbreeding org:anic in- 

 dividuals and the equally general manifestations of sexual and other 

 diversification inside specific lines, as due to the same requirement of 

 protoplasmatic 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 na- 

 ture. For example, the genus Sphagnum, which very rarely produces 

 sjjores, offers a multiplicity of varieties nowhere approached among 

 mosses having normal se.xual reproduction; but, notwithstanding so 

 many differences in minute details, Sphaginnn has remained a very 

 compact, unprogressive group. Cross-fertilization prevents this type 

 of diversification, but it need not on that account l)e 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 multi- 

 plication by fission is another partial and misleading view which has 

 contributed nuich toward the concealment of the causes of evolution. 

 The division of cells is the only method of organic increase; conjuga- 

 tion is not multiplication, but serves as a preliminary stimulant to 

 the necessary cell division. What is growth, for example, among the 

 filamentous alga', composed of chains of cells, is reproduction among 

 the unicellular species where divided cells become separate indi- 

 viduals. Only among the simplest organisms, if anywhere, is indefi- 

 nite reproduction possible without the assistance of conjugation. 



The new science of cytology has made us aware that the division of 

 cells is not a passive or a simple process, but is extremely active, com- 

 plex, and varied. Living protoplasm is in motion, and the discovery 

 that cell walls are not hermetically closed, but are perforated by 

 delicate protoplasmic strands, lends strength to the belief of some 

 biologists that protoplasm circulates, not only inside the individual 

 cells, but through the entire organism. Conjugation may signify 

 that such a circulation extends also throughout the species. Or, to 

 vary the analogy, the net-like structure of protoplasm may be thought 

 of as continuous, not only in the individual, but as binding together 

 the whole species by the intercrossing of the lines of individual 

 descent. As individual organisms will in different degrees endure 



