ON SEXUAL SELECTION IN BIRDS. 413 



are due to a surplus vital energy, and their development to the 

 preference shown by the female for the healthiest male, then how 

 is it possible that health and vigour alone could have developed 

 that wonderful harmony of colour, &c, which we call beautiful? 

 Birds might have developed into ungainly creatures, our most 

 beautiful songsters might have developed shrieks ; and yet, if 

 Mr. Wallace's theory is accepted, such a development as I have 

 indicated would have served the purpose of natural selection 

 equally as well, in that it would have been just as much a sign to 

 the female of health and vigour in the male. From the beauty 

 of the colours previously referred to, and from the harmony with 

 which variations tend to develop, it is evident that there is in 

 nature some direct power continually at work fostering and 

 developing all that is beautiful. As touching on the question of 

 song, it is a curious fact that I have never heard the male Black- 

 cap while mating attempt to mimic other species ; yet, during 

 July, I have heard him mimic — and mimic the Nightingale ex- 

 ceedingly well. It must be as great — probably a greater — effort 

 on the part of a bird to mimic than to sing its natural song, and 

 therefore, in this case, it seems to point to some other power, 

 beyond mere vigour and health, immanent in the bird, which 

 gives it control over its vocal powers, and in this respect it sup- 

 ports my general argument. 



What, then, is the explanation of this display ? For some 

 years I have been impressed with the exceeding variation in 

 plumage, and song, amongst individuals of the same species, and 

 I have come to regard these variations as coincident with 

 the age of the individual. I cannot call to mind any instance 

 amongst our common species in which this variation does not 

 to some extent occur. A few examples will illustrate my mean- 

 ing. The vinous tint on the breast of the Whitethroat varies 

 considerably ; the older males possess it in a marked degree, 

 while in some of the younger birds it can with difficulty be 

 traced ; the older males also have brighter grey on the head and 

 neck, and much more intense fawn on the flanks. The male 

 Grasshopper-Warbler is generally described as having dark 

 spots on the neck ; the older males do not possess these spots, 

 but have instead a rich fawn-colour ; they also have a much 

 purer white on the belly than the younger bird. 



