THE PRINCIPLE DEDUCED 89 



the deathrate should, in the long run, compensate each 

 other. Therefore we may express the facts in the follow- 

 ing generalisation : 



The inherited potential degree of fertility of a species 

 in comparison with that of another species will bear an 

 inverse proportion to its capacity for survival under the 

 environment to which it is adapted. 



The validity of this generalisation is almost self- 

 evident. The fact that a species survives in undiminished 

 numbers is conclusive evidence that its rate of reproduc- 

 tion bears a suitable proportion to its capacity for sur- 

 vival under that environment. To illustrate this, we will 

 take the case of the mouse in comparison with that of 

 the brown rat. Although the rat is many times larger 

 than the mouse, their rates of reproduction are approxi- 

 mately similar. The mouse, however, is tolerated where 

 the rat is fiercely persecuted. The former, owing to its 

 smaller size, can find its way into places inaccessible to 

 the latter, is less conspicuous, and can live upon scraps 

 of food which would be utterly insufficient for the rat. 

 As a consequence, the brown rat, in spite of its greater 

 size, strength and cunning, needs as high a rate of repro- 

 duction as the mouse in order to maintain its numbers. 

 Again, the water-rat, although somewhat smaller and 

 weaker than the brown rat, has a much lower rate of 

 reproduction. That this low rate of reproduction is 

 sufficient is shown by its survival in undiminished numbers. 

 It is subject to little persecution, has great powers of 

 evasion owing to its semi-aquatic habits, and lives on 

 vegetable food which it can find in abundance all the 

 year round. 



Thus we see that each species has a normal rate of 

 reproduction approximately suitable to the conditions of 

 its environment. The potential degree of fertility will 



