FROM MONER TO MAN. 585 



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 con- 

 ceived 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 appar- 

 ently 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 accom- 

 plished ? 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 be- 

 comes gradually more complex ; but embryonic development does not 

 differ essentially from the continued development of infancy, child- 

 hood, 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 exist- 

 ence, 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 varie- 

 ties 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 illustra- 

 tion. 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 deli- 

 cate 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," Ilcrbort Spencer, vol. ii. 



