sistence of force—a doctrine which lies at the very foundation of the stately edifice of modern science.
What is there in the whole stupendous drama of evolution, as conceived by the most enthusiastic supporters of the hypothesis, more wonderful or more difficult of comprehension and acceptance than these facts of embryonic development at which we have briefly glanced?
By the simultaneous processes of growth and differentiation, by a gradual increase of complexity and heterogeneousness continued through a considerable period of time, a microscopic speck of apparently structureless protoplasm, undistinguishable by any known test from the germ of any other animal, develops into the most highly endowed organism of which we have any knowledge.
And through what agencies are these remarkable results accomplished? Besides the inherited impulse of growth and development already referred to, there is furnished to this germ a due supply of ready-prepared food; a certain uniform temperature is also secured to it until the time of birth. After that period, its environment becomes gradually more complex; but embryonic development does not differ essentially from the continued development of infancy, childhood, and youth, by which the adult state is reached. The minute speck of simple protoplasm which constitutes the human organism at the beginning of its career is as truly an independent individual as it ever becomes. At this, as at every subsequent stage of its existence, its life and growth and progress depend on the activities of its own tissues, brought into play by the influence of external forces. Then, as always, it receives food from its environment; while the appropriation and assimilation of this food, as well as the elimination of the products of disintegration and waste, are accomplished by means of the same processes of absorption, chemical combination and decomposition, which constitute nutrition at all periods of existence. The embryon lives its own life—a work which can not be delegated to another.
Our next inquiry is in regard to the forces manifested by living bodies. What are the relations between the highly developed varieties of protoplasm which constitute their different tissues and organs and the remarkable functions—muscular action, emotion, volition, etc.—peculiar to animal organisms?
This question will be best answered by means of a familiar illustration. By an appropriate combination of valves and pistons, of wheels and levers, and numerous other contrivances put together in strict conformity with the principles of mechanics, in which the most delicate allowances are made for unavoidable friction, and the attraction of gravitation is either annihilated by counterbalancing weights or turned to account as a source of power, a machine is constructed which strikingly illustrates the importance, not only of the particular
- See "Principles of Biology," Herbert Spencer, vol. ii.