DURATION OF YOUTH IN MAMMALS 47 



the size of a house mouse, and are turned out to shift for themselves 

 when they are thirty -nine days old. They begin to breed when they 

 are less than six months old and are fully grown a few months later. 

 Mice will breed when they are six weeks old and are fully grown at 

 three to four months old. 



I do not think that it is necessary to go on giving any more 

 examples. It is clear that different kinds of mammals pass through 

 very different periods of time in growing to adult life. There is 

 certainly some relation between size and the duration of youth. 

 On the whole it takes a longer time to grow into a big animal than 

 into a small animal. But the relation is not so close that it can be 

 explained in a simple fashion. The youth of civilised man and of 

 the elephant lasts about the same number of years. A common 

 monkey and a lion take about the same time to grow up. The North 

 American beaver and the bison take very nearly tlae same time, 

 although the latter is several hundred times the bulk of the former. 

 Nor is there any part of the processes of nature which might lead 

 us to expect an inseparable link between time and bulk. The 

 different cells and tissues of the individual body grow at different 

 rates, and these rates may change at the call of circumstances that 

 have nothing to do with size. Temperature, moisture, the nature 

 of the food and many other agencies alter, retard or accelerate the 

 pace. There seems to be a very wide range within which the same 

 organs and tissues or the same kinds of animals or plants may grow 

 more quickly or slowly. None the less, it is reasonable to suppose 

 that closely allied animals have more or less similar constitutions, 

 and such a conclusion is supported by many physiological obser- 

 vations. They have similar habits, they react in similar fashions 

 to the same diseases, and betray their community of blood by 

 responding to similar environments in similar ways. And so com- 

 parisons between the duration of youth and the size of the adult are 

 less misleading when they are made inside the various groups. 

 I have shown that on the whole the larger animals of a group take 

 longer to grow up than the smaller animals of the same group. 

 But the parallel is not exact, and there are many exceptions, as, for 

 instance, among the Camivora. On the other hand, the higher, 

 the more intelligent members of a group are usually the larger 

 animals. Here again there are exceptions, but on the whole it is 

 true of living groups and of the total procession of life in the past. 

 Mammals form the highest class of living animals, and amongst 

 mammals are to be found the largest existing members of the animal 



