In the first place, it is an assertion of the unity of life.  And we must not limit the generality of this proposition.  It is not merely a denial of the fixity of species, an assertion that there are no natural kinds so inseparable from one another that each must be the result of a distinct creative act.  It is also an assertion that human life must be treated as a part in the larger whole of organic being, that the mind of man is continuous with animal perception, that moral activity is continuous with non-moral impulse.  And the assertion of the unity of life is at the same time an assertion of the progress of life.  What we call the higher forms are in all cases developments from simpler and lower forms.

Further, the method of this progress has been described.  Herein indeed lay Darwin’s most important achievement.  He detected and demonstrated the operation of a factor hitherto unsuspected.  This new factor to which he drew attention as the chief agent in organic development was called by him ‘natural selection,’ The name has a positive sound and suggests a process of active choice.  But Darwin was fully aware that the process to which he gave this name was a negative and not a positive operation; and as such it was clearly recognised by him.  The name was, no doubt, chosen simply to bring out the fact that the same kind of results as those which man produces by conscious and artificial selection may be arrived at without conscious purpose by the operation of merely natural forces.  Instead of the ‘fit’ being directly chosen or encouraged, what happens is simply that the ‘unfit’ die out or are exterminated, so that room to live and means of life are left for the survivors.

What may be meant by this idea of ’fitness’—­which meets us in the famous phrase that the ‘survival of the fittest’ in the struggle for life is the goal of evolution—­is a question which brings us at once to the consideration of the ethical significance of the theory.  For it seems to lay claim to give both an explanation of progress and an interpretation of what constitutes worth in conduct.

II.

Ethics and evolution.

There are two things which are not always kept distinct,—­what may be called the ‘evolution of ethics’ and the ‘ethics of evolution,’ The former might more correctly be called the evolution of morality,—­the account of the way in which moral customs, moral institutions, and moral ideas have been developed and have come to take their place in the life of mankind.  Clearly these are all features of human life; and, if the theory of evolution applies to human life, we must expect it also to have some contribution to make to this portion of man’s development,—­to the growth of the customs, institutions, and ideas which enter into and make up his morality.