44-0 Evolution and Adaptation 



and disadvantages (that are obvious as pointed out above), 

 nor is it probable that in this way we can hope to get a final 

 answer to our problem. We may begin by examining some 

 of the modern hypotheses that have been advanced in this 

 connection. 



Darwin has brought together a large number of facts 

 which appear to show the beneficial effects of the union of 

 germ-cells from two different individuals. Conversely, it is 

 very generally believed, both by breeders and by some experi- 

 menters, that self-fertilization in the case of hermaphroditic 

 forms leads, in many cases, though apparently not in all, to 

 the production of less vigorous offspring. Darwin's general 

 position is that it is an advantage to the offspring to have 

 been derived from two parents rather than to have come 

 from the union of the germ-cells of the same individual, and 

 he sees, in the manifold contrivances in hermaphroditic ani- 

 mals and plants to insure cross-fertilization, an adaptation for 

 this purpose. 



This question of whether self-fertilization is less advan- 

 tageous than cross-fertilization is, however, a different ques- 

 tion from that of whether nonsexual methods of reproduction 

 are less advantageous than sexual ones. Since some plants, 

 like the banana, have been propagated for a very long time 

 solely by non-sexual methods without any obvious detriment 

 to them, it is at first sight not easy to see what other advan- 

 tage could be gained by the sexual method. The case of the 

 banana shows that some forms do not require a sexual 

 method of propagation. Other forms, however, are so con- 

 stituted, as we find them, that they cannot reproduce at the 

 present time except by the sexual method. In other words, 

 the latter are now adapted, as it were, to the sexual method, 

 and there is no longer any choice between the two methods. 

 The question of whether a non-sexual form might do better 

 if it had another method of propagation is not, perhaps, a 

 profitable question to discuss. 



