384 ON THE TENDENCY OF VARIETIES TO DEPART 
organization and habits of the various species of ani- 
mals, 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 proportionate 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, 
2nd, that the comparative abundance or scarcity of the 
individuals of the several species is entirely due to their 
organization and resulting ‘habits, which, rendering it 
more difficult to procure a regular supply of food and 
to provide for their personal safety 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 consider- 
ation of varieties, to which the preceding remarks 
have a direct and very important application. 
Useful Variations will tend to Increase; useless or hurt- 
ful 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 
