236 POPULATION'S OF THE SEA 



greatly developed examples of adaptive radiation, for there can be 

 no doubt that they are adaptations to the special needs of the 

 animal's life, in the fishes chiefly to the need of gathering food. 

 Equally fantastic adaptational modifications are found in some 

 land animals; the peacock's tail and the feathers of the bird of 

 paradise are examples, and there are many among the insects. 

 These also are clearly adaptive to the animals' biology, but their 

 use is more often in sexual display. In land animals they are 

 occasional and do not occur in considerable proportions of whole 

 groups as they do in the deep-sea fauna. 



The land animals which show these astonishing forms are 

 closely adapted to restricted modes of life. They are, in fact, 

 highly specialized animals, their specializations serving particular 

 aspects of their biology. In most environments specialization is 

 dangerous. If an animal is adapted only to life in a narrow niche, 

 it is exposed to the danger that the niche may cease to be available 

 as the result of environmental change. If it has very specialized 

 habits, the adaptations needed for these may make it inefficient 

 in other ways. If either of these happens, the animal will perish 

 unless it is able to readapt rapidly. Specialization, therefore, will 

 always be checked by selective forces when it becomes too extreme, 

 but up to this point there will be active selection toward greater 

 elaboration of the specialized adaptations since this will lead to 

 greater efficiency for their uses in the biology of the animal. 



The stability of the deep-sea environments and the fact that 

 variation in the environmental conditions is less local than on land 

 will have reduced the dangers of specialization and allowed it to 

 be carried farther. Also, the long and relatively unchanging 

 history of the environment will have given plenty of time for the 

 elaboration of the specializations, even though evolution may have 

 been slower than it would have been in a more variable environ- 

 ment. It is to be noted that on land most of the animals with 

 fantastically developed specializations occur in the tropical 

 forests, probably the most stable and one of the oldest of terrestrial 

 environments. 



The changes in animal biology that constitute evolution depend 

 on reaction between the animal and its environment. It is there- 



