A PRIORI PRINCIPLE. 401 



answer to the inquiries — why do their fertilities differ so 

 enormously, or why do their mortalities differ so enormously ? 

 and how is the general fertility adapted to the general mor- 

 tality in each ? The balancing process we have contemplated, 

 can go on only within moderate limits — must fail entirely in 

 the absence of a due proportion between the ordinary birth- 

 rate and the ordinary death-rate. If the reproduction of 

 mice proceeded as slowly as the reproduction of men, mice 

 would be extinct before a new generation could arise : even 

 did their natural lives extend to fifteen or sixteen years, it 

 would still be extremely improbable that any would for so 

 long survive all the dangers they are exposed to. Con- 

 versely, did oxen propagate as fast as infusoria, the race 

 would die of starvation in a week. Hence, the minor adjust- 

 ment of varying multiplication to varying mortality in each 

 species, implies some major adjustment of average multipli- 

 cation to average mortality. What must this adjustment be? 

 We have already seen that the forces preservative of race 

 are two — ability in each member of the race to preserve 

 itself, and ability to produce other members — power to main- 

 tain individual life, and power to generate the species. 

 These must vary inversely. When, from lowness of organi- 

 zation, the ability to contend with external dangers is small, 

 there must be great fertility to compensate for the conse- 

 quent mortality ; otherwise the race must die out. When, 

 on the contrary, high endowments give much capacity of 

 self-preservation, a correspondingly-low degree of fertility is 

 requisite. Given the dangers to be met as a constant quan- 

 tity ; then, as the ability of any species to meet them must 

 be a constant quantity too, and as this is made up of the two 

 factors — power to maintain individual life and power to mul- 

 tiply — these cannot do other than vary inversely : one must 

 decrease as the other increases. 



It needs but to conceive the results of nonconformity to 

 this law, to see that every species must either conform to it 

 or cease to exist. Suppose, first, a species whose individuals 



