INHERITANCE, VARIATION AND SELECTION. 39 



plexy and consumption, the tendency is for them to appear at cer- 

 tain definite ages in both parent and offspring. 



In discussing this subject Darwin concludes, 35 that where 

 characters first appear late in life in one sex they tend to be trans- 

 mitted to that sex alone, while those which appear early in life, or 

 before maturity, tend to be transmitted alike to both sexes. The 

 general truth of this rule will be apparent by considering the rela- 

 tionship of adult males and females to their young. When the 

 males and females resemble each other they usually both resemble 

 the young, but when the males and females differ markedly from 

 each other they usually differ in those characters which appear late 

 in life. It is also a general rule that when the adults differ from 

 the young the adult male differs more than the adult female. 



INHERITANCE AT EARLIER AGES. 



When there is a variation from the general rule that characters 

 tend to appear in parent and offspring at corresponding ages, it 

 seems that they more often tend to appear at an earlier age rather 

 than at a later one. Professor Hyatt has assumed that the earlier 

 appearance of a character is a law of nature, and has laid down 

 what is called the law of acceleration or tachygenesis. He says: 

 "All modifications and variations in progressive series tend to ap- 

 pear first in the adolescent or adult stages of growth, and then to 

 be inherited in successive descendants at earlier and earlier stages, 

 according to the law of acceleration, until they become embryonic 

 or are crowded out of the organization and replaced in the devel- 

 opment by characteristics of later origin." 36 While I have no doubt 

 as to the general truth of this law, I am inclined to think that it 



(35) Ibid.. Vol. I, p. 276. 



(36) Smithsonian Contributions to Knowledge, No. 673. 



