38 THE LAW OF BIRTHS AND DEATHS 



sufficient to maintain its numbers in the environment 

 under which it lives. On the other hand, it is a necessary 

 condition of progressive evolution that the reproductive 

 capacity should diminish as the survival-capacity of the 

 organism increases. In other words, that the birthrate 

 should diminish as the deathrate diminishes. For, except 

 under exceptional circumstances, where there is ample 

 opportunity for expansion, it is obviously impossible to 

 have a high birthrate and a low deathrate. If the birth- 

 rate does not diminish, then the deathrate cannot, in the 

 long run, diminish either. On the other hand, where 

 the deathrate is high, the birthrate must be high also, 

 or the species will become extinct. "The condor 1 lays 

 a couple of eggs and the ostrich a score, and yet in the 

 same country the condor may be the more numerous 

 of the two ; the fulmar petrel lays but one egg, yet it 

 is believed to be the most numerous bird in the world. 

 One fly deposits hundreds of eggs, and another, like the 

 Hippobosca, a single one ; but this difference does not 

 determine how many of the two species can be supported 

 in a district. A large number of eggs is of some import- 

 ance to those species which depend on a fluctuating 

 amount of food, for it allows them to rapidly increase 

 in number. But the real importance of a large number 

 of eggs or seeds is to make up for much destruction at 

 some period of life, and this period in the great majority 

 of cases is an early one. If an animal can in any way 

 protect its own eggs or young, a small number may be 

 produced, and yet the average stock be fully kept up ; 

 but if many eggs or young are destroyed, many must 

 be produced, or the species become extinct." 2 



In short, it is a necessary condition of the success of 

 the whole evolutionary scheme that the birthrate and 

 1 See p. 170. * Origin of Species, Darwin, chap. iii. 



