50 CHILDHOOD OF ANIMALS 



smaller birds-of-prey, such, for instance, as turkey- vultures, are 

 full grown at two years old. Fowls and pheasants are full grown 

 at the end of their second year, whilst flamingoes, which are much 

 larger birds, take less than two years to become adult. The dura- 

 tion of youth in birds is therefore remarkably constant ; it varies, 

 from- about one year to nearly four years. There is very little relation) 

 between size and the length of youth. As the intelligence of 

 birds is very remote from that of our own, it is most difficult to 

 estimate which are higher and lower in this respect. But on the 

 whole it must be said that birds are much more instinctive than 

 mammals, that their various duties are performed in a more rigid 

 and mechanical fashion, and that there is therefore less scope than 

 amongst mammals for the experimental period of youth. 



Reptiles live to great ages. They grow very slowly and 

 many of them appear to go on growing throughout their lives. 

 Although there are considerable differences in size between different 

 adult individuals of the same species in both birds and mammals, 

 on the whole it may be said that each species has a rather precisely' 

 limited range of adult size, and that individuals outside the limitsi 

 of this range are abnormal in fact, are dwarfs or giants. The 

 dimensions of the skull or of the body in adults are often so much 

 alike in a large number of individuals that they are useful characters 

 in defining and identifying species. This is not the case with 

 reptiles. No doubt there are limits beyond which crocodiles or 

 pythons do not grow, and there are large lizards and small lizards; 

 large serpents and small serpents. But as compared with birds) 

 and mammals, the different species have not a fixed size. The rate 

 of growth, moreover, is much more dependent on surrounding 

 conditions, particularly on temperature. Birds and mammals have 

 an automatic system of regulating the temperature of their bodies. 

 In our own case, our normal temperature of 98.6 remains nearly 

 constant whether we are exposed to the cold of winter or the heat 

 of summer ; if it goes up a degree or goes down a degree we feel 

 uncomfortable, and if we found it to be 100 or 96 we should 

 know that we were ill and that there was some disorder interfering 

 with the routine of our physiological processes. So also each kind 

 of bird and mammal has its normal temperature, not quite so 

 stable as that of man, but during health kept fairly constant. 

 Reptiles, on the other hand, like batrachians, fishes and probably! 

 most, if not all, invertebrates, have not a normal temperature, but 

 go up and down with the temperature of the air or water with 



