EVOLUTION NOT THE ORIGIN OF SPECIES. 453 



higher animals plants are but loose and imspecialized aggregations of 

 cells, and yet among them also sexual differentiation has made great 

 progress, and in some orders contrivances to insure cross-fertilization 

 are highly developed. 



The extent to which conjugation exists among the lower groups is 

 not yet determined. That it may be omitted for many generations of 

 a simple organism should not be taken to mean that it is entirely absent 

 or has no importance, since among the higher animals, where cross- 

 fertilization is recognized as indispensable, the growth of the body to 

 maturity 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 

 confirmatory exception, in view of the obvious degeneration 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 mod- 

 erately 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 sym- 

 basis, 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 bread- 

 fruit, apple and banana. Such progress is, however, slow and halting, 

 and is accompanied by a decline in reproductive fertility. Symbasis 

 not only sustains the vitality of organisms 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 ac- 

 cumulated. 



To symbasis is due also the arrangement of organisms in the co- 

 herent groups called species, or what may be termed the specific consti- 

 tution of life. Conjugation is the means of symbasis, as division is of 

 reproduction. Sexual and other dimorphism, and the numerous spe- 

 cializations, devices and instincts by which cross-fertilization is secured, 

 are aids to symbasis, just as the spore-sacs, ovaries and placentfe facili- 

 tate reproduction. The phenomena of reproduction and those of sym- 



* There is also the possibility that they secure from their hosts proto- 

 plasmic compounds of high complexity which serve as a partial substitute for 

 conjugation. It is further to be observed that under a kinetic theory the 

 existence of sexual reproduction and cross-fertilization in many fungi in which 

 these processes are still unknown may be inferred from the simple fact that 

 the individuals are grouped into well defined species. 



