cations of structure, is a sealed book to us, and will for ever remain so. The molecular mechanics of crystallization, of chemical processes, seems, it is true, to be more easily understood than that of cells; still, for the present it is as hidden from us as is the latter, though its unintelligibility is of a different kind. It is the adaptation seen in the development and in the functions of the cell which, even were we acquainted with the descent of all forms, would still leave organic nature a mystery. By laws of structure alone we can not explain adaptation in organic beings. Hence, however complete our doctrine of descent, the ancient riddle which has confronted mankind from the beginning persists with all its original obscurity, unless something else comes to succor us. The sphinx of teleology still threatens unconquered from her crag. What boots it to know the reason why all vertebrates are made up of the self-same homologous parts, if we do not further know what natural cause so transformed these parts as to make them exactly answer to the purposes of each separate species? If, to explain this latter fact, a supernaturalistic intervention is still necessary, then we are yet in about the same old rut. Formerly the question used to be why, in repeated creative acts, Omnipotence always clung to the same models, and at times did only indifferent work? But now we must ask why it should, in advance, have tied its own hands, committing itself to faulty constructions and making it impossible for itself, e. g., to create a vertebrate with six extremities, though in a given case such a plan might be a very serviceable one. Hence we are, on the whole, no better off, and have only altered the form of the problem, without coming nearer to a solution.
In these straits we find in the doctrine of natural selection a measurably acceptable solution. Associated with the laws of structure, it would forthwith enable us to understand why organized beings are so wonderfully adapted to one another and to the world around them; why in themselves they are adapted to these ends, at the same time, however, exhibiting many an inadaptation; why they always stand in groups made up of the self-same parts, as though Nature had not been able to invent something new, while nevertheless each one of these parts is cleverly so transformed as to answer a special purpose in each species. Sexual selection, then, comes in to perfect the weapons, offensive and defensive, of the wooing male animal, and furnishes the answer to the question how animated nature happens to lavish plumage adornment on birds; whereas Maupertuis's theorem of the smallest action precludes any superfluity in inanimate nature. Even the glowing hue of Alpine flowers is accounted for by the attraction which brighter-colored individuals exercise upon the insects scarce in those heights, and necessary for fertilization. And mimicry, a fact brought to light by Mr. Alfred Russel Wallace, to whom we owe an important share in the discovery of the grand principle of natural selection, still further multiplies the conditions under which new forms come into existence and
