298 
THE PRINCIPLES OF 
Part IL 
were to vary at birth or soon afterwards, in some 
manner which at this age was of no service to them, 
the chance of the preservation of such variations would 
he small. We have good evidence under domestication 
how soon variations of all kinds are lost, if not selected. 
But variations which occurred at or near maturity, and 
which were of immediate service to either sex, w r ould 
probably be preserved ; as would similar variations 
occurring at an earlier period in any individuals which 
happened to survive. As this principle has an impor- 
tant 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 
wrould 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, whilst 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 instance in brightness, and which were not 
of the least service to the young, would run a good 
chance of being utterly lost. Whilst similar variations, 
which occurring at or near maturity in the compara- 
tively few individuals surviving to this age, and which 
immediately gave an advantage to certain males, by 
rendering them more attractive to the females, would be 
likely to be preserved. No doubt some of the varia- 
tions in brightness which occurred at an earlier age 
would by chance be preserved, and eventually give to 
the male the same advantage as those which appeared 
later ; and this will account for the young males com- 
monly partaking to a certain extent (as may be ob- 
served with many birds) of the bright colours of their 
