THE SPECIES 265 



all arguments levelled against it rebound without effect ; it 

 is nothing but the embodiment of the impulse by the human 

 will to get rid, by every possible means, of plan in Nature. 

 The idea of evolution has thus become the sacred conviction 

 of thousands, but has ceased to have anything to do with 

 unprejudiced investigation of natural phenomena. 



INCREASE IN COMPLEXITY 



We turn again to the question, " On what is based the 

 increase in the complexity of animal forms, which in the course 

 of ages has come about upon the earth ? " 



As has been shown, this increase cannot be explained by 

 the splitting up of species. So far as we can judge at present, 

 this splitting up is merely the consequence of growth in 

 complexity within the confines of a species, to such an extent 

 that it becomes incapable of holding together any longer. 



The next question is whether this growth in the complexity 

 of the completed form of the phenotype within the species 

 can be adequately explained by increase in the mixing of 

 genes in its fixed genotype. 



There is no doubt that the variational extent of the pheno- 

 type, given by combining the genes in every way possible, is 

 very great indeed : and indeed, it may be that this enrich- 

 ment of the phenotype without change of the genotype may 

 lead to the formation of races and" to the splitting off of new 

 species. What is certain is that every new species arising ^ 

 in this way must be poorer in genes than was the mother- 

 species. The result of each splitting off is an increase in 

 specialisation, combined with a decrease in the variational 

 range, and this conditions a greater stability in form pro- 

 duction. 



In the splitting of species we seem to have a division in 

 which there is unequal distribution of the heritable properties, 



