It ON THE TENDENCY OF VARIETIES, ETC. 27 
and, in extreme cases, become altogether extinct. Between 
these extremes the species will present various degrees of 
capacity for ensuring the means of preserving life; and it is 
thus we account for the abundance or rarity of species. Our 
ignorance will generally prevent us from accurately tracing 
the effects to their causes; but could we become perfectly 
acquainted with the organisation and habits of the various 
species of animals, and could we measure the capacity of each 
for performing the different acts necessary to its safety and 
existence under all the varying circumstances by which it is 
surrounded, we might be able even to calculate the pro- 
portionate abundance of individuals which is the necessary 
result. 
If now we have succeeded in establishing these two points 
—Ist, that the animal population of a country is generally 
stationary, being kept down by a periodical deficiency of food, and 
other checks ; and, 2d, that the comparative abundance or scarcity 
of the individuals of the several species is entirely due to their 
organisation and resulting habits, which, rendering it more difficult 
to procure a regular supply of food and to provide for their personal 
safety1 in some cases than in others, can only be balanced by a 
difference in the population which have to exist in a given area— 
we shall be in a condition to proceed to the consideration of 
varieties, to which the preceding remarks have a direct and 
very important application. 
Useful Variations will tend to Increase ; useless or hurtful 
Variations to Diminish 
Most or perhaps all the variations from the typical form 
of a species must have some definite effect, however slight, on 
the habits or capacities of the individuals. Even a change of 
colour might, by rendering them more or less distinguishable, 
affect their safety; a greater or less development of hair 
might modify their habits. More 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 in- 
1 ‘And that of their offspring”’ should have been added. But it must 
be remembered that the writer had no opportunity of correcting the proofs of 
this paper. 
