SEXUALITY A MECHANISM OF EVOLUTION. 15 



That there are two unicelhilar stages in the life history of an orpmisni 

 should not l)e allowed to introduce any confusing- technicalities. For 

 genealogical pur})oses the spore is quite as nuich the descendant of the 

 antherozoid and the egg cell as it would be if the other tissues of the 

 sporoph^'te had not been intercalated. From chromosome redu(^tion 

 to chromosome reduction, from spore to spore, or from egg to egg is 

 one generation, and not two. The prothallus is no more m3^sterious 

 than an}'- other piece of ancient history. The ferns were originally 

 liverworts, the capsules of which had the good fortune to get roots 

 into the ground and keep on growing, but they have not yet learned 

 to dispense with their tirst vain attempt. at building a structure on a 

 simple-celled basis. 



SEXUALITY A MECHANISM OF EVOLUTION. 



Stress has also been laid upon this supposed alternation of "sexual" 

 and " asexual" generations in the belief that a clew was here to be 

 gained regarding the natiu'e of sex and of attendant "mechanisms of 

 heredity." But since only one generation is really involved instead of 

 two, and since the phase of existence which has been termed asexual 

 is in realit}^ the more strongly sexual, it is not surprising that these 

 expectations have not been realized. Sexuality facilitates interbreed- 

 ing and makes it the more effective by distributing new variations 

 throughout the species; it is, in short, a mechanism of diversification and 

 of evolution, a fundamental and universal fact which stands squarely 

 in the way of the alleged law of heredity under which organisms would 

 breed true and be exactl}^ alike. This notion of a uniform and Tuichang- 

 ing heredity," or of any natural tendency to such a condition of organic 



most effective type of organization. The persistence of a clearly two-phased condi- 

 tion in the vascnlar cryptogams and of a reduced alternation of phases, even in the 

 highest algae and flowering plants, is a proof of the extreme slowness of the evolu- 

 tionary i)rogress of the plant kingdom. Animals seem to have passed through the 

 dipliase period of their existence before the dawn of geologic history, and appear in 

 the oldest fossil-bearing strata, not only as completely double-celled organisms but 

 highly differentiated ones at that. Not only are there no traces of the two-phased 

 progenitors which must have gone before the lowest known fossil organisms, but up to 

 now zoologists have not realized the need of postulating such forms at all, and have 

 been content to derive the higher animals from merely simpler but always com- 

 pletely double-celled ancestors, which, of course, are not primitive. It seems not 

 improbable that the completely double-celled condition has been reached inile- 

 pendently by different groups of higher animals, just as it has been approximated, 

 though not attained, ]>y the Fucacese and the phanerogams among plants. The 

 animal kingdom does not contain, so far as is now known, a single species that 

 shows alternating phases in its life history; it has no counterparts of all that wealth 

 of forms ^-hich in the plant kingdom bridge the interval from the protophytes to 

 the flowering plants. 



«" The modifications introduced into palingenesis by kenogenesis are vitiations, 

 strange, meaningless additions to the original, true course of evolution." — Haeckrl, 

 Evolution of Man, vol. 11, p. 460, note 9. 



