590 THE PHYSIOLOGY OF REPRODUCTION 



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 about the age of twenty-five years." The fecundity 

 of the average individual woman may be described, therefore, 

 as forming a wave which, starting from sterihty, rises somewhat 

 rapidly to its highest point, and then gradually falls again to 

 sterility. 



There can be no doubt that animals as a general rule tend 

 to foUow a similar law. A dog generally has fewer puppies in 

 its first htters than afterwards, while in its decUning 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 obviously many individual exceptions. Geyehn ^ 

 gives the following table showing the fertihty of the domestic 

 fowl at different ages : — 



First year after hatching 15 to 20 

 Second „ „ . 100 „ 120 



Third „ „ 120 „ 135 



Fourth „ „ . 100 „ 115 



Fifth „ „ . 60 „ 80 



Sixth year after hatching 50 to 60 

 Seventh „ „ . 35 „ 40 



Eighth „ „ . 15 „ 20 

 Ninth „ „ . 1 „ 10 



Furthermore Minot* observed that in guinea-pigs the size 

 of the litters increased with age during the first sixteen months 

 of their lives. 



Effects of Environment and Nutrition 



That the generative system in animals is peculiarly susceptible 

 to changed conditions of existence has been recognised from 

 early days. Thus Aristotle ^ commented on the increased 

 fertihty of sheep in a favourable environment. In more recent 



' 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. 621). " Duncan, loc. cit. 



' Geyelin, Poultry-Breeding in a Gonwnercial Point of View, London, 

 1865. 



* Minot, " Senescence and Rejuvenation," Amer. Jour, of Phys., vol. 

 xii., 1891. 



* Aristotle, History of Animals, Bohn's Library, London. 



