28 
NATURAL SELECTION 
XI 
habit. It is also evident that most changes would affect, 
either favourably or adversely, the powers of prolonging 
existence. An antelope with shorter or weaker legs must 
necessarily suffer more from the attacks of the feline carni- 
vora ; the passenger pigeon with less powerful wings would 
sooner or later be affected in its powers of procuring a regular 
supply of food ; and in both cases the result must necessarily 
be a diminution of the population of the modified species. If, 
on the other hand, any species should produce a variety 
having slightly increased powers of preserving existence, that 
variety must inevitably in time acquire a superiority in 
numbers. These results must follow as surely as old age, in- 
temperance, or scarcity of food produce an increased mortality. 
In both cases there may be many individual exceptions : but 
on the average the rule will invariably be found to hold good. 
All varieties will therefore fall into two classes — those which 
under the same conditions *would never reach the population 
of the parent species, and those which would in time obtain 
and keep a numerical superiority. Now, let some alteration 
of physical conditions occur in the district — a long period of 
drought, a destruction of vegetation by locusts, the irruption 
of some fresh carnivorous animal seeking “ pastures new ■ — 
any change in fact tending to render existence more difficult 
to the species in question, and tasking its utmost powers to 
avoid complete extermination, — it is evident that, of all the 
individuals composing the species, those forming the least 
numerous and most feebly organised variety would suffer 
first, and, were the pressure severe, must soon become extinct. 
The same causes continuing in action, the parent species would 
next suffer, would gradually diminish in numbers, and with 
a recurrence of similar unfavourable conditions might also 
become extinct. The superior variety would then alone 
remain, and on a return to favourable circumstances would 
rapidly increase in numbers and occupy the place of the 
extinct species and variety. 
Superior' Varieties will ultimately Extirpate the original Species 
The variety would now have replaced the species , of which 
it would be a more perfectly developed and more highly 
organised form. It would be in all respects better adapted 
