INDEFINITELY FROM THE ORIGINAL TYPE. 35 
might modify their habits. Moré important changes, 
such as an increase in the power or dimensions of 
the limbs or any of. the external organs, would 
more or less affect their mode of procuring food 
or the range of country which they could inhabit. 
It is also evident that most changes would affect, 
either favourably or adversely, the powers of pro- 
longing existence. An antelope with shorter or 
weaker legs must necessarily suffer more from the 
attacks of the feline carnivora; 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 neces- 
sarily 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 su- 
periority. Now, let some alteration of physical 
conditions occur in the district—a long period of 
drought, a destruction of vegetation by locusts, the 
D2 
