INTRODUCTION xv 



be assumed, according to this author, that only a 

 qualitative change in the nutritive fluid of the germ- 

 cells could produce an effect : a quantitative change 

 would only cause increased or decreased nourish- 

 ment of the entire germ cells. 



In my own volume on Sexual Dimorphism in the 

 Animal Kingdom, published in 1900, I attempted to 

 explain the limitation of secondary sexual characters 

 not only to one sex, but usually to one period of the 

 individual life, namely, that of sexual maturity; 

 and in some cases, as in male Cervidae, to one season 

 of the year in which alone the sexual organs are active. 

 It had been known for centuries that the normal 

 development of male sexual characters did not take 

 place in castrated animals, but the exact nature of 

 the influence of the male generative organs on that 

 development was not known till a year or two later 

 than 1900, when it was shown to be due to an 

 internal secretion. My argument was that all 

 selection theories failed to account for the limitation 

 of secondary sexual characters in heredity, whereas 

 the Lamarckian theory would explain them if the 

 assumption were made that the effects of stimulation 

 having been originally produced when the body and 

 tissues were under the influence of the sexual organs 

 in functional activity, these effects were only 

 developed in heredity when the body was in the same 

 condition. 



About the year 1906, when preparing two special 

 lectures in London University on the same subject, 

 I became acquainted with the work of Starling and 

 others on internal secretions or hormones, and saw 

 at once that the hormone from the testes was the 

 actual agent which constituted the ' influence ' 

 assumed by me in 1900. In these lectures I 



