282 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



differences in the required direction, and thus to create new 

 characters. In our example it ought to be able, after pre- 

 serving those individuals with a colour nearest to the required 

 shade, to lead their descendants onward through successive 

 stages towards a complete harmony of colour. 



But such a result is quite unattainable with the asexual 

 method of reproduction : in other words, natural selection, in 

 the true meaning of the term, viz. a process which could 

 produce new characters in the manner above described, is an 

 impossibility in a species propagated by asexual reproduction. 



If it could be shown that a purely parthenogenetic species 

 had become transformed into a new one, such an observation 

 would prove the existence of some force of transformation 

 other than selective processes, for the new species could not 

 have been produced by these latter. As already explained, the 

 only selection which would be possible for such a species, 

 would lead to the survival of one group of individuals and to 

 the extinction of all others. Thus in our example that group 

 of individuals would alone survive, the ancestors of which 

 originally possessed the appropriate colour. But if one group 

 alone survived, it follows that all hereditary individual dif- 

 ferences would have disappeared from the species, for the 

 members of such a single group are identical with one another 

 and with their original ancestors. We thus reach the conclu- 

 sion that monogonic reproduction can never cause hereditary 

 individual variability, but that, on the other hand, it is very 

 likely to lead to its entire suppression. 



But the case is very ditlerent with sexual reproduction. 

 When once individual differences have begun to appear in 

 a species propagated by this process, uniformity among its 

 individuals can never again be reached. So far from this 

 being the case, the differences must even be increased in the 

 course of generations, not indeed in intensity, but in number, 

 for new combinations of the individual characters will con- 

 tinually arise. 



Again, assuming the existence of a number of individuals 

 which differ from one another by a few hereditary individual 

 characters, it follows that no individual of the second genera- 

 tion can be identical with any other. They must all differ, not 

 only actually but also potentially, for their differences exist at 



