288 TDE TRINCirLES OF [Part II. 



for tills general and remarkable coincidence between the 

 period of variability and that of sexual selection — princi- 

 ples wliicli are quite indejiendent of each other? I think 

 "wc 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 offsprmg 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 aftervv^ard, in some manner Avhich 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 jirobably be preseiwed ; 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 assi;me 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 Vtiiich vrere 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 



