CORRELATED VARIATIONS. 93 



in captivity. This was at Worthing, where the typical 

 male form is the only one known, so that the brood ob- 

 tained were most certainly produced from a cross of the 

 two varieties. The offspring were sharply divided into 

 10 male and 8 female betularia } and 6 male and 5 fe- 

 male doubledayaria. 



Conversely to the modification of a community in 

 one direction by reason of the increased productiveness 

 of certain of its members, we may experience modifica- 

 tion in the opposite direction by reason of a decreased 

 productiveness. For instance, Dr. Beddoe * states that 

 there is a good deal of evidence as to the greater 

 liability of blonds than of brunets to certain classes 

 of disease. At least this is so in America, as has been 

 shown by Baxter, f Thus it would appear that the 

 blonds have less chance than the brunets of contribut- 

 ing their due proportion to the next generation, and so 

 they must be relatively diminishing in numbers. That 

 this is the case is supported by the fact that of Ameri- 

 cans accepted for service in the army a greater propor- 

 tion were brunets than of the English, Irish, and 

 Germans accepted. Thus: 



Among the Americans were 66 light and 34 dark complexioned 



" " English " 70 " " 30 " 



" Irish " 70 " " 30 " " 



" Germans " 69 " " 29 " 



The fact that most species are to some extent mu- 

 tually sterile, whilst their hybrid offspring are almost 

 invariably so, proves that the physiological condition of 



* Science Progress, vol. v. p. 384, 1896. 



f" Medical Statistics of the Provost-Marshal-General's Bureau," 

 Washington, 1875. 



