EVOLUTION 201 



the chromosomes, distributed as required by the breeding 

 facts of Mendelian heredity, are the machinery for im- 

 pressing the finer details. There is very little to be said 

 for this point of view, though it may have use as a working 

 hypothesis. But granting its truth, it does not detract 

 from the benefits gained by the origin of sex ; the major- 

 ity of variations are comparatively small, changes in 

 detail, the very kind which are known to be Mendelian 

 in their inheritance. 



Yet sexual reproduction in itself does not assure these 

 advantages, though they are based upon it. There must 

 be means for the mixture of germ plasms. This oppor- 

 tunity was furnished originally by bisexuality. After- 

 wards hermaphroditism was tried ; and, though manifestly 

 an economic gain, it was, on the whole, unsuccessful except 

 as functional bisexuality was restored by self-sterility, 

 protandry, protogyny or mechanical devices which pro- 

 moted cross-fertilization. The prime reason for the suc- 

 cess of sexual reproduction, then, as Weismann 209 first 

 maintained, though he knew not the exact reason, is the 

 opportunity it gives for mingling germ plasms of different 

 constitutions, and thereby furnishing selective agencies 

 many times the raw material producible through asexual 

 reproduction. It was not sexual reproduction per se 

 which triumphed, but exogamy. 



While increased variability and the greater elasticity 

 in adaptiveness to new environments thus gained must be 

 given the first consideration when seeking the significance 

 of sex, they are not the only advantages. As we have 

 seen, an increased size, greater viability and increased 

 production of offspring commonly result from crossing 

 somewhat different forms. Here is a combination of 



