From the preceding discussion two things may be inferred: first, the dependence of pleasure and pain on the subject-activity, whether the activity be that of perception or of what is specifically called action; and secondly, the modification of pleasure and pain, and transition from one to the other, along with the modification of that subject-activity. To the application of both these conclusions there may be limits; but their general accuracy does not seem doubtful. What the doctrine of evolution adds to this is its proof of the indefinite modifiability of human function. "It is an essential principle of life," Mr Spencer wrote,[188] before he had arrived at his general theory of evolution, "that a faculty to which circumstances do not allow full exercise diminishes; and that a faculty on which circumstances make excessive demands increases;" and to this we must now add, "that, supposing it consistent with maintenance of life, there is no kind of activity which will not become a source of pleasure if continued; and that therefore pleasure will eventually accompany every mode of action demanded by social conditions."[189] It is, he holds, a "biological truth," that "everywhere faculties adjust themselves to the conditions of existence in such wise that the activities those conditions require become pleasurable."[190] The vast periods of time over which evolution stretches are scarcely needed to show how pleasure may be made to follow from almost any course of action consistent with the continuance of life. The change of habits which often takes place in the history of a nation, and even in the life of an individual, makes this sufficiently obvious. But, if we still think of making attainment of pleasure the end of conduct, the doctrine of evolution must give us pause. It has been already argued that, given certain sources of, and susceptibilities for, pleasure, the course of evolution has not been such as to produce an exact coincidence between them and the actions which further life. But it would seem that, given habits of acting which are consistent with the conditions of life, and which are systematically carried out, these will not fail to grow pleasant as the organism becomes adapted to them. At the best, it is difficult enough to say, even for the individual, whether one imagined object or course of action will exceed another in pleasurable feeling or not. But, when we remember that function and feeling may be modified indefinitely, it is impossible to say what course of conduct will produce the greatest amount of pleasure for the race. Taking in all its effects, we cannot say that one way of seeking pleasure is better--that is, will bring more pleasure--than another. Bearing in mind the modifications which evolution produces, it seems impossible to guide the active tendencies of mankind towards the goal of greatest pleasure, except by saying that the greatest pleasure will be got from the greatest amount of successful, or of