is so loose and elastic that a working arrangement between the parts
can be reached in all sorts of directions. The lesser systems are so
far self-governing that they can be trusted to get along in almost
any combination; though of course some combinations are naturally
stronger and more stable than the rest, and hence tend to outlast them,
or, as the phrase goes, to be preserved by natural selection.

It is time to take account of the principle of natural selection. We
have done with the subject of variation. Whether use and disuse have
helped to shape the fresh forms of life, or whether these are purely
spontaneous combinations that have come into being on what we are
pleased to call their own account, at any rate let us take them as
given. What happens now? At this point begins the work of natural
selection. Darwin's great achievement was to formulate this law;
though it is only fair to add that it was discovered by A.R. Wallace
at the same moment. Both of them get the first hint of it from Malthus.
This English clergyman, writing about half a century earlier, had shown
that the growth of population is apt very considerably to outstrip
the development of food-supply; whereupon natural checks such as
famine or war must, he argued, ruthlessly intervene so as to redress
the balance. Applying these considerations to the plant and animal
kingdoms at large, Darwin and Wallace perceived that, of the
multitudinous forms of life thrust out upon the world to get a
livelihood as best they could, a vast quantity must be weeded out.
Moreover, since they vary exceedingly in their type of organization,
it seemed reasonable to suppose that, of the competitors, those who
were innately fitted to make the best of the ever-changing
circumstances would outlive the rest. An appeal to the facts fully
bore out this hypothesis. It must not, indeed, be thought that all
the weeding out which goes on favours the fittest. Accidents will