n ON ANIMAL LIFE 51 



the beauty of animals and plants are not only 

 thus a delight to the eye, but afford us also 

 some of the most interesting problems in 

 Natural History. Some probably are not 

 in themselves of any direct advantage. 

 The brilliant mother-of-pearl of certain shells, 

 which during life is completely hidden, 

 the rich colours of some internal organs of 

 animals, are 'not perhaps of any direct 

 benefit, but are incidental, like the rich and 

 brilliant hues of many minerals and precious 

 stones. 



But although this may be true, I believe 

 that most of these colours are now of some 

 advantage. " The black back and silvery 

 belly of fishes ' ' have been recently referred to 

 by a distinguished naturalist as being obvi- 

 ously of no direct benefit. I should on 

 the contrary have quoted this case as one 

 where the advantage was obvious. The dark 

 back renders the fish less conspicuous to an 

 eye looking down into the water ; while the 

 white under-surface makes them less visible 

 from below. The animals of the desert are 

 sand-coloured ; those of the Arctic regions are 



