38 SEXUAL DIMORPHISM 



conception of it, whatever its mechanism, is a tendency in 

 the new individual to pass successively through the same 

 stages of growth as its parent. That which is inherited is 

 not a state, but a process. Unisexual characters consist, in 

 general, in modifications of growth in particular parts of the 

 body, very often in excessive growth or hypertrophy. They 

 may, in fact, in many cases, be correctly described as ex- 

 crescences, and these excrescences are as truly the result in 

 the first instance of mechanical or other irritation, as a corn 

 in the human epidermis. But the irritation and the con- 

 sequent hypertrophy or proliferation have been in every 

 generation inseparably associated with a certain condition of 

 the organism, that condition, namely, in which the testes, or 

 the ovaries in certain cases, were mature and active. 



If there were no question of inheritance in the matter, 

 and the secondary sexual character were produced de novo by 

 certain stimulations applied only in one sex when the animal 

 began to breed, then we should have no cause to wonder why 

 such characters were confined to one sex and to one period 

 of life in that sex. The reason would be that the stimulations 

 only acted on the individuals of the one sex at that period. 

 The characters would be acquired characters confined to the 

 individuals which acquired them, and not appearing until the 

 actions and influences to which they were due came into 

 operation. 



But it is evident enough that the development of the 

 characters at the present stage of evolution takes place by 

 heredity, even though the usual irritations are partly or 

 entirely wanting. The modifications of growth which in 

 each generation have been produced in a certain phase of 

 individual development, occur of their own accord when that 

 phase is reached or approached. In the more familiar cases 

 the phase in question is the functional activity of the testes, 

 and the physiological condition associated therewith. 



