24 DARWINISM AND RACE PROGRESS. 



animals, we have only to enumerate the number of 

 the progeny produced by each pair, which is often 

 prodigious, and knowing as we do that the number 

 of individuals in a species remain virtually the same 

 in a given district for long periods of time together, 

 we conclude that the room of the parents is just 

 filled by a younger pair, and all the excess of their 

 progeny over and above this one pair must have 

 succumbed to surrounding want and hardship. To 

 give one concrete example out of hundreds that 

 might be selected, let us take the case of the golden 

 eagle given by Weismann in his essay on the " Dura- 

 tion of Life." He says : " Let us fix the duration of 

 life in the golden eagle at sixty years, and its period 

 of immaturity (of which the length is not exactly 

 known) at ten years, and let us * assume that it lays 

 two eggs a year, then a pair will produce one 

 hundred eggs in fifty years, and of these only two 

 will develop into adult birds, and thus on an average 

 a pair of eagles will only succeed in bringing ,a pair 

 of young to maturity once in fifty years ; and so far 

 from being an exaggeration, this calculation rather 

 under-estimates the proportion of mortality among 

 the young." 



But in all probability most of us are more conver- 

 sant with the ways of the domesticated cat than with 

 those of the golden eagle. The cat produces its first 

 litter of three or four before it is a year old. Its 



