48 DEVELOPMENT AND HEREDITY. 



refer to the fact that the parts of the same animal 

 which receive less light develop less pigment. 

 Thus there is less colour on the under side of all quad- 

 rupeds, snakes, fishes, crawling insects, crustaceans, 

 worms, mollusks, echinoderms, and sea-anemones. 

 Among many of the flying insects and many birds 

 this condition is less marked or absent for the obvious 

 reason that while in the air they are exposed to a 

 nearly equal light on all sides. Further, it is a mat- 

 ter of general observation that animals which during 

 their lives are not exposed to light do not develop 

 pigment. Such animals, whether they live as par- 

 asites buried in the flesh or in the alimentary canal 

 of other animals, or whether they live in caves 

 where no light penetrates, remain colourless, often 

 semi-transparent ; and if they have eyes no pigment 

 develops in them. This has been explained as the 

 result of natural selection, which is supposed to act 

 in such a way as to prevent the waste of the energy 

 that would be necessary to produce pigment where 

 it is not necessary to the life of the organism. But 

 the facts just mentioned are so striking and uni- 

 versal, that it is difficult to imagine how natural 

 selection could have acted to produce the same 

 result in all cases under such varying conditions, 

 especially when we remember the prodigal way in 

 which energy is wasted in producing ornamental 



