THE INSURGENCE OF LIFE 131 



only great fertility, but rapid coming to maturity. Mr. 

 Newton Miller has supplied precise data as to the fertility 

 of the brown rat in captivity, and these are of serious human 

 interest because of the importance of this animal as a 

 destroyer of food-supplies and a disseminator of disease. 

 The creature breeds all the year round, and five or six 

 litters may be actually reared by a pair in the course of a 

 year. If the young are destroyed or removed at birth, 

 there may probably be a litter every month. In one case 

 seven litters were produced in seven months by one female. 

 The young are carried from 23J-25J days before birth. 

 The number in a litter varies from six to nineteen, with an 

 average between ten and eleven. They are not full grown 

 before eighteen months, but both sexes are ready for 

 reproduction not later than the end of the fourth month. 



The rabbit may have six young ones in a litter, and four 

 litters in a year ; and the young may begin to breed when 

 they are six months old. This rate is far surpassed by 

 some of the mice, and when we descend to the level of 

 insects and the like we find an extraordinarily rapid suc- 

 cession of generations. In the time required for the pro- 

 duction of one generation of a larger higher animal, the 

 lower type has had many generations and has produced 

 an enormously greater weight of living matter. It was 

 this that led Linnaeus to say that three flies consume the 

 carcass of a horse as quickly as a lion (' Tres muscae con- 

 sumunt cadaver equi, aeque cito ac leo '). 



Huxley calculated that if the descendants of a single 

 green-fly all survived and multiplied, they would at the end 

 of one summer weigh down the population of China. The 

 descendants of a common house-fly would in the same time 

 six generations of about three weeks each occupy a 



