CROSSING. 



AN examination of the causes for variability, for geno- varia- 

 tion, shows that there are two out-standing ones, spontaneous 

 geno- variation or mutation, and the recombination of genes 

 through crossing in the widest sense. We will examine mutation 

 as a cause for that geno- variation which can furnish the mater- 

 ial for evolution, and see that, as there is a good reason to 

 assume that mutation. ever consists of anything but the spon- 

 taneous loss of a gene, it cannot be considered as of any real 

 general importance for the evolution of new species. This 

 brings us to the question whether crossing, re-combination of 

 genes made possible by heterozygosis, is in itself sufficient to 

 cause the genotypix variation which is necessary for evolution. 



In the chapter on Heredity we have given the reasons for 

 our assumption that in. those cases, where the difference 

 between two alternative characters is due to one gene more or 

 less, dominance means presence, and recessiveness means ab- 

 sence. And here a difficulty comes in. If we see individuals 

 which have new characters, dominant over the corresponding 

 characters of the patent stock, we must admit that this means 

 that these individuals possess genes which were lacking in the 

 parent specie?. Now there are several instances where such new 

 characters are not found in any other organism, or at least not 

 in closely related organisms. Should we not admit that the cor- 

 responding genes have been acquired spontaneously, that their 

 origin has been a special creation ? Davenport, among others, 

 has answered this question in the affirmative. As in some tame 

 chickens there exist genes which are absent from the genotype 

 of Callus bankiva, the alleged progenitor of all domestic chick- 

 ens, positive mutation, the spontaneous acquisition of new 



