288 THE PRINCIPLES OF [Part II. 



for this general and remarkable coincidence between the 

 period of variability and that of sexual selection — princi- 

 ples which are quite independent of each other ? I think 

 we can see the cause: it is not that the males have never 

 varied at an early age, but that such variations have com- 

 monly been lost, while those occurring at a later age have 

 been preserved. 



All animals produce more offspring than can survive 

 to maturity ; and we have every reason to believe that 

 death falls heavily on the weak and inexperienced young. 

 If, then, a certain proportion of the offspring were to vary 

 at birth or soon afterward, in some manner which at this 

 age was of no service to them, the chance of the preser- 

 vation of such variations would be small. We have good 

 evidence under domestication how soon variations of all 

 kinds are lost, if not selected. But variations which oc- 

 curred at or near maturity, and which were of immediate 

 service to either sex, would probably be preserved ; as 

 would similar variations occurring at an earlier period in 

 any individuals which happened to survive. As this prin- 

 ciple has an important bearing on sexual selection, it may 

 be advisable to give an imaginary illustration. We will 

 take a pair of animals, neither very fertile nor the reverse, 

 and assume that after arriving at maturity they live on an 

 average for five years, producing each year five young. 

 They would thus produce 25 offspring ; and it would not, 

 I think, be an unfair estimate to assume that 18 or 20 out 

 of the 25 would perish before maturity, while still young 

 and inexperienced ; the remaining seven or five sufficing 

 to keep up the stock of mature individuals. If so, we can 

 see that variations which occurred during youth, for in- 

 stance, in brightness, and which were not of the least ser- 

 vice to the young, would run a good chance of being 

 utterly lost. While similar variations, which occurring 

 at or near maturity in the comparatively few individuals 



