Now to my mind the simplest and clearest and most profound philosophy that was ever written upon this subject is to be found in the 2d and 3d chapters of Mr. Darwin's 'Descent of Man.' In these chapters it appears that just as most physical characteristics of organisms have been evolved and preserved because they were useful to the individual in the struggle for existence against other individuals and other species, so this particular feeling has been evolved and preserved because it is useful to the tribe or community in the struggle for existence against other tribes, and against the environment as a whole. The function of conscience is the preservation of the tribe as a tribe. And we shall rightly train our consciences if we learn to approve those actions which tend to the advantage of the community in the struggle for existence. There are here some words, however, which require careful definition. And first the word purpose. A thing serves a purpose when it is adapted to some end; thus a corkscrew is adapted to the end of extracting corks from bottles, and our lungs are adapted to the end of respiration. We may say that the extraction of corks is the purpose of the corkscrew, and that respiration is the purpose of the lungs. But here we shall have used the word in two different senses. A man made the corkscrew with a purpose in his mind, and he knew and intended that it should be used for pulling out corks. But nobody made our lungs with a purpose in his mind, and intended that they should be used for breathing. The respiratory apparatus was adapted to its purpose by natural selection--namely, by the gradual preservation of better and better adaptations, and the killing off of the worse and imperfect adaptations. In using the word purpose for the result of this unconscious process of adaptation by survival of the fittest, I know that I am somewhat extending its ordinary sense, which implies consciousness. But it seems to me that on the score of convenience there is a great deal to be said for this extension of meaning. We want a word to express the adaptation of means to an end, whether involving consciousness or not; the word purpose will do very well, and the adjective purposive has already been used in this sense. But if the use is admitted, we must distinguish two kinds of purpose. There is the unconscious purpose which is attained by natural selection, in which no consciousness need be concerned; and there is the conscious purpose of an intelligence which designs a thing that it may serve to do something which he desires to be done. The distinguishing mark of this second kind, design or conscious purpose, is that in the consciousness of the agent there is an image or symbol of the end which he desires, and this precedes and determines the use of the means. Thus the man who first invented a corkscrew must have previously known that corks were in bottles, and have desired to get them out.