INTRODUCTION 37 



The facts being so, there is rivalry, combat, and competition, 

 and there must be selection. But the important truth, 

 which appears to have been generally overlooked, is that in 

 the case of each special organ its special employment subjects 

 it to special, usually mechanical, irritation or stimulation, to 

 which other organs of the body are not subjected. Every 

 naturalist and every physiologist admits that in the indi- 

 vidual any irritation or stimulation regularly repeated pro- 

 duces some definite physiological effect, some local and special 

 change of tissue in the way of either growth or absorption, 

 enlargement or decrease, or change of shape. Thus not only 

 hypothetically at some former time, but actually at present 

 in every individual, the unisexual organs or appendages are 

 subjected in their functional activity to special strains, con- 

 tacts, and pressures, that is, to stimulation, which must and 

 does have some physiological effect on their development and 

 mode of growth. This argument will, of course, be received 

 with more or less courteous contempt, because it is held by 

 the majority that any effects on the individual, any " acquired 

 characters," are not inherited, and therefore have nothing to 

 do with the evolution of characters which are constant in 

 species, and evidently hereditary. I do not propose here to 

 attempt to prove that acquired characters are inherited. My 

 object is merely to point out how remarkably the multitudi- 

 nous facts all agree with the hypothesis that secondary sexual 

 characters are due to the inheritance of acquired characters. 



"We have seen that the attempts that have been made to 

 explain the restriction of such characters not only to one 

 sex, but to the period of maturity in the individual and often 

 to one period of the year, have not been successful. The 

 true explanation in my opinion is, that heredity causes the 

 development of acquired characters for the most part only in 

 that period of life and in that class of individuals in which 

 they were originally acquired. Heredity, according to my 



