40 CHILDHOOD OF ANIMALS 



than those often given to monkeys in captivity, I am inclined to think 

 that they were not varied enough, nor exciting enough for the 

 normal rate of growth. The small American monkeys, such as mar- 

 mosets, become full grown in from two to three years. 



The length of the period of youth thus becomes shorter and 

 shorter as we descend from the highest human types to the lowest 

 monkeys, and is parallel with some other qualities of this group of 

 animals. The potential longevity, the age to which an animal can 

 attain under the most favourable conditions, is greatest in the higher 

 races of man, where it may be a century, seldom exceeds fifty or 

 sixty years in the lower races of man, and, so far as the somewhat 

 scanty evidence at our disposal goes, decreases as we pass down the 

 scale of monkeys from the man-like apes to the simplest little 

 monkeys. It cannot be said, however, that there is any definite 

 proportion between the length of youth and the length of the whole 

 life, in the fashion that the Greeks supposed the height of the head 

 to be a definite proportion of the total height. The span of a com- 

 plete life is not divided according to any ideal rule or law into so 

 many parts for helpless infancy, so many for aspiring youth, and 

 so many for maturity. Each portion varies with the particular needs 

 of the particular species, and no more is to be expected than that 

 the mode of division should be rather more alike amongst species 

 that are nearly related, and rather less alike amongst species that 

 are far separate. 



There is also a rough correspondence between the duration of youth 

 and the size of the creatures in the man -monkey group. A full-grown 

 male gorilla, it is true, is larger, although not taller, than a finely 

 built man, but the human race as a whole consists of larger and finer 

 animals than the anthropoid apes, whilst these in their turn exceed 

 the baboons, which exceed the ordinary monkeys of India and 

 Africa, and so on down to the tiny marmosets. It is tempting 

 to suppose that it must take longer to grow into a big animal than 

 into a little animal. This also is true only when nearly related 

 creatures are compared. Mere increase of bulk tells us little. A 

 mushroom grows much more quickly than a daisy, a gooseberry 

 and a huge vegetable marrow take nearly the same time to swell 

 out. A human child takes nearly two hundred days to double its 

 weight at birth, whilst new-born mice quadruple their weight in 

 twenty-four hours. The nature of the organism, the complexity 

 of its structure and the particular conditions under which 

 it lives must aU be taken into account, and are of more 



