110 THE METHOD OF DARWIN. 



that sex. " I was led to infer that a relation of 

 this kind exists from the fact that whenever 

 and in whatever manner the adult male has 

 come to differ from the adult female, he differs 

 in the same manner from the young of both 

 sexes." 1 



The principle that characters appearing at an 

 early age are inherited by both sexes, and char- 

 acters appearing late in one sex are restricted 

 to that sex, is an induction from certain strik- 

 ing differences between the adult males and 

 females, and between the adult male and the 

 young of both sexes in many species. By 

 itself, without verification or other deductive 

 bracing, it would have been an interesting gen- 

 eralization. But Darwin sought to strengthen 

 it both by observing whether it held true in 

 particular cases, and by deducing it from more 

 general laws. In the first of the two quotations 

 given above, he pointed out the probability 

 that the truth of the principle depends on the 

 known changes that take place in the constitu- 

 tion of the sexes on approaching maturity. It 

 occurred to him to put the principle to a crucial 

 test, and to rely on the result. Thereupon fol- 

 lowed the investigation on the deer family, 

 described elsewhere in more detail. 



1 Descent cf Man, etc., p. 276. 



