The preceding considerations have made it evident that the idea of Evolution has undergone a broadening process since Darwin first brought it before the world. It is necessary to glance briefly at some of the chief phases and the general significance of this process in order to define the extent and intent of the concept as far as science has made such definition possible. To Darwin himself the struggle for existence was always between the unities represented by complete organisms whether as isolated individuals, or in family, tribal, or national groups. Everywhere in his calculations, appearing unchanged in his results, is found the unknown quantity of variation from ancestral type, the known factors being heredity, and natural and sexual selection in the struggle for existence. Wallace's ideas as to color in birds deprive the theory of sexual selection of one of its most important points of application in Darwin's work. It is, in fact, easy to see that sexual selection cannot neutralize natural selection, that any particular form of sexual selection can arise and finally survive only by a harmony with the direction of natural selection, and that the two must therefore appear, even from any standpoint of freedom of the will, as continually attaining coincidence. It has been said, above, that the struggle for existence was, for Darwin, between the organisms as unities. This consistent position of the specialist has been criticised, from a more general point of view, by Lewes in his essay on the Nature of Life,[96] in which he asserts that we must logically "extend our conception of the struggle for existence beyond that of the competition and antagonism of organisms--the external struggle; and include under it the competition and antagonism of tissues and organs--the internal struggle." "Mr. Darwin," he says, "has so patiently and profoundly meditated on the whole subject, that we must be very slow in presuming him to have overlooked any important point. I know that he has not altogether overlooked this which we are now considering; but he is so preoccupied with the tracing out of his splendid discovery in all its bearings, that he has thrown the emphasis mainly on the external struggle, neglecting the internal struggle; and has thus, in many passages, employed language which implies a radical distinction where--as I conceive--no such distinction can be recognized. 'Natural Selection,' he says, 'depends on the survival, under various and complex circumstances, of the best-fitted individuals, but has no relation whatever to the primary cause of any modification of structure.'[97] On this we may remark, first, that selection does not _depend_ on the survival, but _is_ that survival; secondly, that the best-fitted individual survives because of that modification of its structure which has given it the superiority; therefore, if the primary cause of this modification is not due to selection, the selection cannot be the cause of species.