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 always happen.  On the whole, however, the type that is most at home under the surrounding conditions, it may be because it is more complex, or it may be because it is of simpler organization, survives the rest.

Now to survive is to survive to breed.  If you live to eighty, and have no children, you do not survive in the biological sense; whereas your neighbour who died at forty may survive in a numerous progeny.  Natural selection is always in the last resort between individuals; because individuals are alone competent to breed.  At the same time, the reason for the individual’s survival may lie very largely outside him.  Amongst the bees, for instance, a non-working type of insect survives to breed because the sterile workers do their duty by the hive.  So, too, that other social animal, man, carries on the race by means of some whom others die childless in order to preserve.  Nevertheless, breeding being a strictly individual and personal affair, there is always a risk lest a society, through spending its best too freely, end by recruiting its numbers from those in whom the engrained capacity to render social service is weakly developed.  To rear a goodly family must always be the first duty of unselfish people; for otherwise the spirit of unselfishness can hardly be kept alive the world.