THROUGH THE YEAR 101 



the cause is in the immense strength of the sun 

 there : the interference colour, given strong light, 

 is perhaps easier to produce, sooner produced, than 

 the ordinary pigment for the scale of butterfly and 

 the feather of bird. Also it is iribr-e ; effective-^ 

 a tour de couleur indeed. J * jJ t! > j v ' 



I doubt not these ornaments, ;wh?^h?r of 'pigrp&iit,\ 

 or iridescence, have been worked up for utility- 

 the survival of the fittest through beauty. Beauty 

 is the bait. The female butterflies and birds have 

 chosen from the finest liveried of their suitors, and 

 the finest liveried have proved through the ages the 

 fittest. 



That has been the law for an unthinkable period 

 of time. But there may well have been an overflow 

 of the beauty by which sexual selection has done the 

 work of evolution. Not an overflow in colour, for 

 that branch of beauty seems to be all devoted to 

 these ends of utility, but an overflow in another 

 branch the branch of song in birds. I have long 

 felt that, though song in the main is to attract the 

 female a bait, like that of colour a certain amount 

 of it has overflowed into other channels. The winter, 

 autumn and late summer singing of birds is often, 

 I think, not sexual. The delicious little singing of 

 the willow-warbler in August and September ere he 

 leaves for Africa is not to attract the hen bird. 



