FERTILITY 589 



year elapses before the hare begins to breed. These two factors 

 compounded result in a difference of fertility far greater than 

 can be ascribed to unlikeness of the two creatures in size." 



Furthermore, Spencer refers to the case of the bat, which 

 has been already mentioned as being abnormally unprolific in 

 proportion to its size. The relatively low rate of multiplication 

 is of course ascribed to a relatively high rate of expenditure 

 resulting from the habit of flying. 



In a similar way Spencer explains such well-known facts as 

 that hens cease to lay when they begin to moult. " While they 

 are expending so much in producing new clothing, they have 

 nothing to expend for producing eggs." 



There can be little doubt that Spencer's generalisation is 

 in the main true, but it is equally certain that it cannot be 

 applied indiscriminately to explain the relative degrees of fertility 

 in all animals, and consequently it must not be pressed too far. 

 Some of the more special factors which control fertility are 

 referred to below, and it is evident that many (though not all) 

 of these fall within the scope of Spencer's generalisation. 



The rate of increase as distinguished from the rate of repro- 

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

 factors, 'of which the rate of reproduction is only one. 



EFFECT OF AGE 



Matthews Duncan l has discussed at some length the varia- 

 tion 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 commence- 

 ment 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 

 1 Duncan, loc. cit. 



