48 CHILDHOOD OF ANIMALS 



kingdom. In the age of Reptiles, when they were the lords of 

 creation, the largest existing animals were reptiles. In the age of 

 Batrachians the largest existing creatures were batrachians. A ad so 

 inside the orders of living mammals, they are, on the whole, the most 

 highly organised creatures that have been able to increase in size. 

 Certainly there are many advantages in being big. A bulky animal 

 can resist changes in temperature better than a smaller creature, 

 which may be more quickly overheated or chilled through. A big 

 animal, other things being equal, is more powerful and can protect 

 itself better and travel greater distances than a smaller animal of the 

 same kind. But there are also great disadvantages. A big animal 

 needs more food than a smaller one, and can less easily escape 

 the observation of its enemies. The struggle for existence is 

 specially keen among animals with similar habits and structure, 

 and amongst these it is the more highly organised and intelligent 

 that can become large with least risk. Amongst mammals I do 

 not doubt but that the apparent connection between the duration 

 of youth and the size is secondary ; both depend on intelligence 

 It is the more intelligent animals that have the longest period of 

 youth. 



In the beginning of this chapter I spoke of the lengthening of the 

 period of youth in our own case, even in our own time. Breeders 

 of domesticated animals have found that they can prolong or shorten 

 the duration of youth in the case of farm stock. There are many 

 instances showing that wild animals in captivity mature more 

 quickly in some cases, more slowly in other cases, than their fellows 

 under natural conditions. The series of animals in the different 

 orders of mammals show that there is an increase in the duration of 

 youth as we pass from the lower forms to the higher forms. Putting 

 these different sets of observations together, we must draw the conclu- 

 sion that the rate of growth in animals has been altered in the course 

 of evolution, and in such a fashion as to prolong youth in 

 the higher forms. This lengthening of youth is not completely 

 explained by increase of size, nor even by increased complexity of 

 structure. Its advantage is that it gives the opportunity for education 

 in the widest sense of the word, a space for experiment and for the 

 replacing of instinct by intelligence. 



