24 
RATE OF INCREASE 
kind remains fairly constant from year to year, it is evident 
that a vast amount of destruction must take place, and that 
only a few of the offspring of each generation survive. The 
destruction is effected by animal enemies, parasitic enemies, 
competition with other individuals of the same or different 
kinds, unfavourable conditions of climate or environment, 
&c. When a species is transferred to a new country the 
balance that existed in its former habitat between its rates 
of increase and destruction is interfered with, often in a 
favourable direction, as is illustrated by the multiplication 
of rabbits in Australia, of Elodea in Britain, of cardoons on 
the Pampas. On the other hand many species are unable 
to establish themselves in new countries, even though, as 
far as we can judge, the conditions of life are as favourable 
as in their old localities. 
Struggle for Existence. It follows that there must 
be among plants and animals a continual struggle for 
existence and propagation, both against one another and 
against the external conditions of life, keener between in- 
dividuals of the same kind than between those of different 
kinds, but occurring wherever individuals are so close to- 
gether as to compete for soil, light, air, or other necessaries. 
There may even be competition between plants far apart, 
for the services of insects. In deserts, arctic countries, and 
similar situations, the struggle is rather between plant and 
climate or environment, than between plant and plant, as 
the individuals are not usually close together. 
We have now to consider what determines which indi- 
viduals are to succeed, and which to perish. 
Natural Selection. If there were no variation among 
the individuals of a species, there would be a ‘survival of 
the accidentally favoured,’ but when variation enters into 
the problem the result is different. Those individuals which 
are superior to their fellows in any character or sum of 
characters (not necessarily all favourable) that is of im- 
portance in the general conditions of life, eg. by giving 
increased economy or efficiency, or by protecting against 
risk or danger, will on the whole be the survivors, and there 
will thus be a survival of the fittest , or a 7iatural selection , 
analogous to the artificial selection employed by breeders, 
who select in each generation the finest and most suitable 
