“Progress having once begun by the establishment of species, the laws of atavism and variability will suffice to tell the remainder of the story.  The colonies gifted with the faculty of forming others in their likeness will soon by their increase become sole masters of the field; but the common enemy being thus destroyed, the struggle for life will be renewed among the conquerors.  The saying that ’a house divided against itself cannot stand,’ receives in Nature its flattest contradiction.  Civil war is here the very instrument of progress; it brings about the survival of the fittest.  Original differences in the cell-colonies, however slight, will bring about differences of life and action; the latter, continued through successive generations, will widen the original differences of structure; innumerable species will thus spring up, branching forth in every direction from the original stock; and the competition of these species among each other for the ground they occupy, or the food they seek, will bring out and develop the powers of the rivals.  One chief cause of superiority will lie in the division of labor instituted by each colony; or, in other words, in the localization of the colony’s functions.  In the primitive associations (as in the lowest organisms existing now), each cell performed much the same work as its neighbor, and the functions necessary to the existence of the whole (alimentation, digestion, respiration, etc.) were exercised by every colonist in his own behalf.  Social life, however, acting upon the cells as it acts upon the members of a human family, soon created differences among them—­differences ever deepened by continuance, and which, by narrowing the limits of each colonist’s activity, and increasing his dependence on the rest, rendered him fitter for his special task.  Each function was thus gradually monopolized; but it came to be the appanage of a single group of cells, or organ; and so excellent did this arrangement prove, so greatly were the powers of each commonwealth enhanced by the division of its labor, that the more organs a colony possessed, the more likely it was to succeed in its struggle for life. . .  We shall go no further, for the reader will easily fill out the remainder of the picture for himself.  Man is but an immense colony of cells, in which the division of labor, together with the centralization of the nervous system, has reached its highest limit.  It is chiefly to this that his superiority is due; a superiority so great, as regards certain functions of the brain, that he may be excused for having denied his humbler relatives, and dreamed that, standing alone in the centre of the universe, sun, moon, and stars, were made for him.”

Let us learn from the same writer how both eyes of the flounder get, quite unintentionally, on the same side of the head.  The writer makes much of this case (see p. 306), and we are not disposed to pass it by: