PREFACE 



[t must be admitted that my discussion of Sexual Dimor- 

 phism in this work is incomplete, and this in two senses. 

 In the first place, evidence of the kind I have endeavoured to 

 collect is in many cases wanting, and has yet to be obtained 

 in the future. The absence of such information is partly 

 due to the difficulty of obtaining it, to the want of oppor- 

 tunities, and partly to the neglect of opportunities and to 

 the absence of any special efforts to obtain it. It is not 

 my intention by such an assertion to throw the blame upon 

 collectors and field naturalists. Their methods of work are 

 naturally adapted to the ends they have in view, and so long 

 as the minute study of habits and conditions of life is gener- 

 ally regarded as of little importance, so long will the collection 

 of specimens and the discovery of species be considered the 

 chief objects of a zoological expedition. It is strange that even 

 from the point of view of selection more attention has not been 

 devoted since Darwin's time to the study of the particular 

 functions or utilities of special characters. When we desire 

 to know fully the irritations and stimulations produced on 

 living tissues by the special habits of the animal, and how its 

 special organs or characters are affected when they come into 

 action, it is clear that the detailed study of the habits of the 

 living animal becomes still more important. A zoological 



