CHAPTER X 



THE EOLE OF INBEEEDING AND OUTBREEDING 



IN EVOLUTION 



In our brief consideration of the more important 

 changes which have occurred in the reproductive mechan- 

 isms of animals and plants, several features stand out 

 impressively. Both animals and plants have followed 

 modes of reproduction that are identical in what are 

 deemed to be the essential features, something which can 

 be said of no other life process. It is not enough simply 

 to say that sexual reproduction has become the dominant 

 mode of propagation among organisms. One must go 

 further. Cross-fertilization, either continuous or occa- 

 sional, is the really successful method of multiplication 

 everywhere. Such a parallel evolution in the two king- 

 doms is valid evidence of real worth in the process : a 

 consideration of the evidence on inbreeding and cross- 

 breeding permits us to state this value in concrete terms. 



The establishment of methods of reproduction which 

 maintain variation and inheritance mechanisms on a high 

 plane of efficiency is naturally a fundamental requirement 

 in evolution. Since, however, we have seen that there is 

 no reason for believing sexual reproduction to be better 

 adapted to assure a numerous progeny than asexual re- 

 production, it either must be a more perfect means of 

 hereditary transmission, or it must offer selective 

 agencies a greater variety of raw material. 



Fortunately we are able to eliminate the first alter- 

 native. There is definite evidence that sexual reprodnc- 



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