626 THE PHYSIOLOGY OF REPRODUCTION 



and it is evident that many (though not all) of^hese fall within the 

 scope of Spencer's generalisation. 



The rate of increase as distinguished from the rate of reproduction 

 (in any given species) depends upon a large number of factors, of 

 which the rate of reprqduction is only one. 



Effect of Age 



Matthews Duncan ^ has discussed at some length the variation 

 which occurs in the fertility of women according to their age. He 

 adduces statistical evidence showing that the fertility of the female 

 population increases gradually from the commencement of the child- 

 bearing period of life until about the age of thirty, and then it 

 gradually declines. He shows also that the fertility is much greater 

 before the climax is reached (at thirty years) than after it is passed. 

 These conclusions, however, apply merely to the actual productiveness 

 (i.e. the number of births), as opposed to the capability of bearing 

 children, which Duncan designates the fecundity. By eliminating 

 from his calculations all women not living in married life, Duncan 

 arrives at the following conclusions, which are based on statistics 

 showing the productiveness of wives : ^ (1) " That the initial fecundity 

 of women gradually waxes to a climax, and then gradually wanes " ; 

 (2) " That initial fecundity is very high from twenty to thirty-four 

 years of age " ; and (3) " That the climax of initial fecundity is 

 probably tfbout the age of twenty-five years." The fecundity of the 

 average individual woman may be described, therefore, as forming 

 a wave which, starting from sterility, rises somewhat rapidly to its 

 highest point, and then gradually falls again to sterility. 



Pearl * refers to the case of a ewe whose complete breeding record 

 conforms to this description very closely. In her first two years she 

 had single lambs, in her third year twins, then for six years in 

 succession triplets, for the next six years twins, and finally for two 

 more seasons single lambs. In the next two years which were the 

 last of her life this ewe did not produce any lambs. 



There can be no doubt that the majority of animals tend to 

 follow a similar law. A dog generally has fewer puppies in its first 

 litters than afterwards, while in its declining years there is a 

 diminution until sterility is reached once more. The same is said to 

 be the case with the bear, the elk, and other animals,* but there are 



' Duncan, loc. cit. 



2 It is, of course, obvious that it is impossible to determine statistically the 

 real " fecundity " (using the term as defined by Duncan) in view especially of 

 the practice of volitional interference with conception (see below, p. 659). 



3 Pearl, "Note Regarding the Relation of Age to Fecundity," Science, 

 vol. xxxvii., 1913. 



* Duncan, loc. cit. 



