THE FACTORS OP OEGANIC EVOLUTION. 57 



units largely as survival of the fittest has been instru 

 mental in furthering and controlling the combination of 

 these units into visible organisms, and eventually into large 

 ones ; yet we must ascribe to the direct effect of the medium 

 on the first forms of life, that character of which this 

 every where- operative factor has taken advantage. 



Let us turn now to another and more obvious attribute of 

 higher organisms, for which also there is this same general 

 cause. Let us observe how, on a higher platform, there 

 recurs this differentiation of outer from inner how this 

 primary trait in the living units with which life commences, 

 re-appears as a primary trait in those aggregates of such 

 units which constitute visible organisms. 



In its simplest and most unmistakable form, we see this 

 in the early changes of an unfolding ovum of primitive 

 type. The original fertilized single cell, having by spon 

 taneous fission multiplied into a cluster of such cells, there 

 begins to show itself a contrast between periphery and 

 centre ; and presently there is formed a sphere consisting 

 of a superficial layer unlike its contents. The first change, 

 then, is the rise of a difference between that outer part 

 which holds direct converse with the surrounding medium, 

 and that inclosed part which does not. This primary 

 differentiation in these compound embryos of higher 

 animals, parallels the primary differentiation undergone by 

 the simplest living things. 



Leaving, for the present, succeeding changes of the 

 compound embryo, the significance of which we shall have 

 to consider by-and-by, let us pass now to the adult forms 

 of visible plants and animals. In them we find cardinal 

 traits which, after what we have seen above, will further 

 impress us with the importance of the effects wrought on 

 the organism by its medium. 



From the thallus of a sea-weed up to the leaf of a highly 

 developed phaenogam, we find, at all stages, a contrast 



