246 APPENDIX III LETTERS PUBLISHED IN NATURE. 



others maintain it to be absolutely essential. This latter view has 

 arisen from an exaggerated opinion as to the power of intercrossing to 

 keep down any variety or incipient species and merge it in the parent 

 stock." (Darwinism, p. 144.) 



I think we shall reach a more consistent and complete apprehension 

 of the subject by starting with the fundamental laws of heredity, and 

 refusing to admit any assumption that is opposed to these principles, 

 till sufficient reasons have been given. Laws which have been estab- 

 lished by thousands of years of experiment in domesticating plants 

 and animals should be, it seems to me, consistently applied to the gen- 

 eral theory of evolution. For example, if in the case of domesticated 

 animals, "it is only by isolation and pure breeding that any specially 

 desired qualities can be increased by selection" (see Darwinism, p. 99), 

 why is not the same condition equally essential in the formation of 

 natural varieties and species? If in our experiments we find that 

 careful selection of divergent variations of one stock does not result in 

 increasingly divergent varieties unless free crossing between the -varieties 

 is prevented, why should it be considered an exaggeration to hold that 

 in wild species ' ' the power of intercrossing to keep down any variety 

 or incipient species, and merge it in the parent stock," is the same that 

 we have found in domestic species. Experience shows that segrega- 

 tion, which is the bringing of like to like in groups that are prevented from 

 crossing, is the fundamental principle in the divergence of the various 

 forms of a given stock, rather than selection, which is like to like through 

 the prevention of certain forms from propagating; and I think we intro- 

 duce confusion, perplexity, and a network of inconsistencies into our 

 exposition of the subject whenever we assume that the latter is the 

 fundamental factor, and especially when we assume that it can produce 

 divergence without the cooperation of any cause of segregation divid- 

 ing the forms that propagate into two or more groups of similars, or 

 when we assume that segregation and divergence can not be produced 

 without the aid of diverse forms of selection in the different groups. 

 The theory of divergence through segregation states the principle 

 through which natural selection becomes a factor promoting some- 

 times the stability and sometimes the transformation of types, but 

 never producing divergent transformation except as it cooperates \\ ith 

 some form of isolation in producing segregation ; and it maintains that 

 whenever variations whose ancestors have freely intergenerated are, 

 from any combination of causes, subjected to persistent and cumula- 

 tive forms of segregation, divergence more or less pronounced must be 

 the result. The laws of heredity on which this principle rests may be 

 given in the three following statements. 



