330 THE COLOURS OF ANIMALS 



affords an explanation of the gradual growth of such 

 a pattern in spite of its heterogeneous elements, an 

 explanation which I do not think is supplied by any 

 other theory. Mr. Wallace has greatly insisted on 

 the amount of individual variation, and we know that 

 variations in the minutest elements of organs must 

 occur as constantly as in the organs themselves. The 

 presence or absence of bubbles of gas and of pigment, 

 the chemical constitution of pigments, the width of 

 spaces in the tissues, are all subject to constant varia- 

 tion, and afford abundant material for the production 

 of any aesthetic effect, if only subjected to selective 

 breeding. And I have endeavoured to show that 

 selection by female preference is now supported by 

 certain striking facts, which were not available when 

 Darwin first argued that this principle has been effi- 

 cient in producing the colours displayed in courtship. 



The unsatisfactory nature of the phrase ' surplus of vital 

 energy ' 



I will only briefly allude to the unsatisfactory nature 

 of such vague phrases as ' surplus of vital energy,' 

 when used to explain the appearance of the definite 

 results which have been described above. The only 

 evidence for such surplus vitality is the excitability of 

 the nervous system, which is correlated with the ac- 

 tivity of the reproductive organs in the breeding sea- 

 son, and which leads to violent and active movements 

 generally forming part of the display in courtship. 



