214 THE BIOLOGY OF BIRDS 



less well-equipped males are either left unmated, or have 

 less numerous and successful families as the result of their 

 matings. (d) As Darwin himself hinted, there is much 

 reason to think that the female who has to be wooed sur- 

 renders herself not to the male who has a particular character 

 in special excellence, but to the one whose tout ensemble has 

 most successfully excited her sexual interest. 



Cunningham'' s Theory. — In his interesting " Sexual Di- 

 morphism " Mr. J. T. Cunningham argues in support of a 

 Lamarckian interpretation. " In either sex unisexual cha- 

 racters have, as a general rule, some function or importance 

 in the special habits or conditions of life of the sex in which 

 they occur." " 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 individual any irritation 

 or stimulation regularly repeated produces 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 in- 

 dividual, the unisexual organs or appendages are subjected 

 in their functional activity to special strains, impacts, and 

 pressures, that is, to stimulation, which must and does have 

 some physiological effect on their development and mode of 

 growth." To explain the restriction of sex-characters to 

 one sex, to the period of maturity, and often to one period of 

 the year, Cunningham supposed 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." Unisexual characters are 

 largely of the nature of excrescences which originated from 

 mechanical or other irritation in the male or the female at 

 particular times and in particular states of body. They are 

 now part and parcel of the inheritance, but they are not 

 expressed in the body except in association with physiological 



