SEXUAL SELECTION 165 



by a cumbrous and costly process to reduce their numbers. 

 What we have to show is first that the individual, even if his 

 safety is endangered by his grand plumage, yet has, because of 

 it, a better chance of leaving offspring, and secondly, that the 

 species gains by the system. 



The explanation, I believe, is really quite simple. The fine The ex- 

 plumes are found in their highest development in polygamous 

 species, and this fact is the key to the problem. culty 



Let us imagine a polygamous species ; in which there are sixty 

 males, thirty of whom are burdened with brilliant plumage, 

 while the attire of the other thirty is less burdensome and less 

 bright. Let us further imagine that half of the brilliant thirty 

 lose their lives through the dangers their beauty brings with it, 

 or through their pugnacity. The surviving fifteen, however, 

 will be the fathers of practically all the next generation, the dull 

 thirty being driven out of the field or being rejected by the hen- 

 birds because of their dulness. Thus, though a particular 

 individual might fall a victim to over-ornamentation, yet the 

 brilliant as a class would have an advantage over the dull, in the 

 sense that they would have more offspring. This must be the 

 case, since a bright-coloured bird will have a good chance of 

 being the father of many, whereas there are heavy odds against a 

 dull-coloured one having any progeny at all. To simplify the pro- 

 blem I have imagined that there are thirty of each sort. In reality 

 they would not, of course, be divisible into classes so definite, 

 but there would probably be a descent by nice gradations from 

 the brightest to the dullest. This, however, does not affect the 

 working of the system, the result of which must be that the 

 number of young a cock-bird leaves behind him will be roughly 

 in proportion to the development in him of the secondary male 

 characters. The species too, so far from being a loser, ivill gain. In 

 each generation the finest of the males will be the sires of 

 practically all the next generation, Sexual Selection thus greatly 

 intensifying Natural Selection. 



We must assume that in every case general vigour accom- Fine 

 panics fine plumage. And this all the evidence goes to prove ; 

 the most brilliant are the best fighters also. In all polygamous comitant 



of vigour 



