286 Evolution and Adaptation 



the questions of variation which were raised in the preceding 

 pages. It should be pointed out, however, that it would be 

 premature to do more than indicate, in the most general 

 way, the application of these conclusions. The chief value 

 of Mendel's results lies in their relation to the theory of in- 

 heritance rather than to that of evolution. 



In the first place, Mendel's results indicate that we cannot 

 make any such sharp distinction as Darwin does between 

 the results of inheritance of discontinuous and of continu- 

 ous variations. As Mendel's results show, it is the separate 

 characters that must be considered in each case, and not 

 simply the sum total of characters. 



The more general objection that Darwin has made may 

 appear to hold, nevertheless. He thinks that the evolution of 

 animals and plants cannot rest primarily on the appearance 

 of discontinuous variations, because they occur rarely and 

 would be swamped by intercrossing. If Mendel's law ap- 

 plies to such cases, that is, if a cross were made between 

 such a sport and the original form, the hybrid in this case, if 

 self-fertilized, would begin to split up into the two original 

 forms. But, on the other hand, it could very rarely happen 

 that the hybrid did fertilize its own eggs, and, unless this oc- 

 curred, the hybrid, by crossing with the parent forms in each 

 generation, would soon lose all its characters inherited from 

 its "sport" ancestor. Unless, therefore, other individuals 

 gave rise to sports at the same time, there would be little 

 chance of producing new species in this way. We see then 

 that discontinuity in itself, unless it involved infertility with 

 the parent species, of which there is no evidence, cannot be 

 made the basis for a theory of evolution, any more than can 

 individual differences, for the swamping effect of intercross- 

 ing would in both cases soon obliterate the new form. If, 

 however, a species begins to give rise to a large number of 

 individuals of the same kind through a process of discontinu- 

 ous variation, then it may happen that a new form may 



