The principles involved in the theory of evolution are, in brief, as follows. In the first place, it shows that there is a tendency, brought about by natural selection, for organisms to harmonise with or become adapted to their environment--a tendency, that is to say, towards unity of organism and environment, and, in so far as external conditions are uniform, towards a general unity of life. In the second place, the theory implies variation in organisms, produced either by the unequal incidence of external forces, or by the spontaneous action of the organism, or by both causes combined. The mere increase in the number of living organisms leads to a modification of the conditions of life by which new variations are encouraged. And this tendency to variation in organisms--not merely the diversity of external environment--is perpetually complicating the conditions which the former tendency, that towards unity, helps to bring into harmony. It thus happens that there is, in the third place, a continual process of readjustment and oscillation between the tendency towards unity and that towards variety, which, through opposition and conciliation, produces continuity in nature. Each newly formed unity between organism and environment is broken by a new variation of the organism or of the environment, which further complicates the problem to be solved by the unifying process, and gives scope for a more intricate and more comprehensive readjustment. Unity, Variety, and Continuity are thus the three principles implied in the theory of evolution.[271] [Sidenote: 2. Unsuccessful application of these principles to ethics;] It is from these principles that the attempt has been made to show the ethical bearing of evolution. The first of them, Unity, is represented in the theory that would make adaptation to environment the end of conduct; and the second is represented ethically in the doctrine suggested by Mr Spencer, that the degree of morality depends on the degree of complexity in act and motive. But both of these views are obviously one-sided, even from the point of view of empirical evolution. Taken together, the principles on which they depend make up that law of continuous and progressive advance which may be regarded as expressing the essential characteristic of the theory. And from this more general and accurate expression of it, we might have expected to have been able to elicit the contribution which evolution has to make to the determination of the ethical end. But after examining the various forms which it may take, we have been unable to obtain from it a principle of action. [Sidenote: (_a_) the principles being treated as derived from experience,]