SEXUAL SELECTION 47 



and of others who have followed him, the results stated above 

 were not without exception. For example, Castle found that 

 a certain proportion of the mice resulting from the first cross 

 of a gray with a white mouse were not gray, as we would 

 have expected according to Mendel's laws, nor yet white, but 

 were a dappled gray and white. In such a case there was a 

 true mingling of the characters of both parents in the young, 

 neither set of characters predominating. 



Enough has been said to show that interbreeding be- 

 tween the different individuals of a species is not promis- 

 cuous and wholly indeterminate, and therefore the favorable 

 varieties preserved by natural selection from among the indi- 

 viduals of any generation will not necessarily be swamped 

 when these divergent forms come to breed. We will return 

 to this subject again. The phenomena of organic nature 

 seem to indicate very clearly that evolution has taken place, 

 and the evidence points strongly to natural selection as a 

 real factor and apparently the chief factor in this evolution. 



But natural selection is not the only factor in evolution. 

 Reference has already been made to sexual selection and 

 segregation, and besides these there is still another important 

 factor, the inheritance of parental modifications. Let us 

 consider these. 



SEXUAL SELECTION 



By sexual selection, as we will use the term, is meant the 

 exercise of choice in mating. 1 Among plants and lower 

 animals, if cross-fertilization occur at all, propinquity at the 



1 Those familiar with Darwin's writings will recognize that I use the phrase 

 sexual selection in a more limited sense than does Darwin, following rather the 

 usage of Wallace, Lloyd Morgan, and others. For example, Darwin includes under 



