22 THE INFANCY OF ANIMALS 



carnivores, for example, are not only not more numerous 

 than they are, but that they did not, long since, increase 

 more rapidly than the animals they preyed upon, and 

 so extinguish both themselves and their victims ages ago. 

 And support is lent to such a view by the undoubted 

 fact that in colder climates an abnormal increase in the 

 food supply is accompanied by an abnormally increased 

 fertility of the animals affected thereby. Thus " lem- 

 ming years " — or seasons when this interesting and 

 remarkable rodent is unusually abundant — are marked 

 by an increase in the numbers of all the predatory birds 

 and beasts which prey upon them : the lemmings them- 

 selves owing their increase to more favourable conditions 

 — warmer weather, more abundant food. This increase 

 of predatory species is not merely the gathering of a 

 crowd to the feast : it is an actual increase in number by 

 a rise in the birth-rate, both the number produced at a 

 birth and the number of families produced during the 

 year being materially increased. 



Such phenomena, however, are rather peculiar to 

 northern latitudes, where we may imagine the maximum 

 potentiality of reproduction only occasionally finds an 

 opening. In hot countries there is no such check on 

 reproduction, and the adjustment between the eaters 

 and the eaten has long since struck a balance. 



Whatever be the factors controlling the number of 

 young produced at a birth, it is certain that we find a 

 wide range in the period of infancy, which is associated 

 with the time necessary to attain maturity, which in 

 turn is related to the bulk of the body. Young mice 

 can scarcely be said to have any childhood, for, as we 

 have already remarked, they may themselves be parents 

 within six weeks of their birth. The elephant does not 



