IN THE THEORY OF NATURAL SELECTION. 275 



so, all existing- species would have been included as variations of 

 the first species. Natural selection must be able to do infinitely 

 more than this, if it is to be of any importance as a principle of 

 development. It must be able to accumulate minute existing- dif- 

 ferences in the required direction, and thus to create new characters. 

 In our example it ought to be able, after preserving those in- 

 dividuals with a colour nearest to the required shade, to lead their 

 descendants onward through successive stag-es 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 pro- 

 duced 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 differences 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 conclusion 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 different with sexual reproduction. When 

 once individual differences have begun to appear in a species pro- 

 pagated 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 continually arise. 



Again, assuming the existence of a number of individuals which 

 differ from one another by a few hereditary individual characters, 



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