THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 409 



cross-fertilization .is recognized as indisf)ensable, the growth of the 

 body to matnrity requires millions of cell divisions, each of which 

 would mean a new generation in a unicellular species. The supposed 

 absence of sexual reproduction in certain parasitic and saprophytic 

 groups is a coufirmatorv exception in view of the ol)vious degenera- 

 tion of such organisms." 



To the many speculations on the purpose of sex and cross-fertiliza- 

 tion it can do no harm to add the conjecture that the presence of 

 moderately diverse qualities of protoplasm facilitates cell division. 

 Some have held that the function of sex is to assist evolution by 

 producing variations, and others that it neutralizes variation by 

 maintaining a stable average. From the kinetic point of view it 

 appears that symbasis, as represented by the phenomena of sex and 

 of cross-fertilization, is not an impediment to evolution nor a device 

 to cause variation, but a means of communicating it. Variations 

 appear without sex, and may even be accumulated, as by the adding of 

 one bud variation to another in plants propagated by grafts or by 

 cuttings, like the breadfruit, apple, and banana. Such progress is, 

 however, slow and halting, and is accompanied by a decline in repro- 

 ductive fertility. Symbasis not only sustains the vitality of organ- 

 isms already evolved, but it is directly responsible for the upbuilding 

 of the complex structure and vital economy of the higher plants and 

 animals, and it builds the faster when by the differentiation of sexes 

 two sets of variations can be accumulated. 



To symbasis is due also the arrangement of organisms in the 

 coherent groups called " species," or what may be termed the specific 

 constitution of life. Conjugation is the means of symbasis, as divi- 

 sion is of reproduction. Sexual and other dimorphism, and the 

 numerous specializations, devices, and instincts by which cross-fer- 

 tilization is secured, are aids to symbasis, just as the spore-sacs, 

 ovaries, and placenta facilitate reproduction. The phenomena of 

 reproduction and those of symbasis are coml)ined, perhaps inextrica- 

 bly, but all attempts at assigning them to a single cause or property 

 have failed. 



Cross-fertilization is connnonly 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 varia- 

 tions. There is the amplest experimental evidence that cross- 

 breeding is necessary to maintain the quality and efficiency of the 



a Under a kinetic theory the existence of sexual reproduction and cross-fertili- 

 zation in many fungi in which these processes are still unl<no\vn may be inferred 

 from the simple fact that the individuals are grouped into well-delined species. 



