6 HEREDITY AND SELECTION IN SOCIOLOGY 



same will be determined by the number of progeny brought forth 

 yearly, and the relation of that number to the number of deaths 

 before maturity is reached. The fecundity of a given species 

 being constant, its death-rate must needs be constant also, if the 

 number of its members is, under given conditions, to remain the 

 same. This relation of the death-rate to the fecundity-rate of 

 a species explains why the normal number of that species remains 

 constant. If we know the fecundity-rate of a species, we can 

 calculate its death-rate ; for, if the normal number of the species 

 is to remain constant, only two ofEspring per pair can, on an 

 average, reach maturity and reproduce, for otherwise the con- 

 stancy of the normal figure of the species would no longer be 

 maintained. 



Thus, to take examples which Weismann has given us,^ a 

 pair of storks brings forth annually four offspring, and continues 

 reproducing at the same rate for twenty years, thus begetting a 

 total of eighty offspring. Of these eighty, only two can attain 

 matiirity and reproduce, while the other seventy-eight must be 

 destroyed before attaining maturity. A trout lays yearly 

 600 eggs, and continues doing so during ten years. Out of the 

 total of 6,000 eggs, 5,998 must be destroyed during development 

 and youth, and only two can reach maturity. In other cases 

 the death-rate is far greater. Certain worms bring forth no less 

 than 100,000,000 eggs, of which 99,999,998 must be destroyed 

 before maturity. 



Thus a constant ratio is maintained between the fecundity 

 rate and the death-rate of a species, and the maintenance of this 

 ratio is essential if the normal number of that species is to remain 

 constant. It may excite wonder that, given the constancy of 

 this ratio, the fecundity-rate should be so excessively high ; 

 and yet, when we come to closer examination, we find that the 

 enormous sum total of births is not superfluous, is not a mere 



* A. Weismann, Vortrage iiher Deszendenztheorie, i. 39. Jena, 

 1904. 



