i 9 4 TJ;}: POPULAR SCIENCE MONTHLY, 



pebbles, no conglomerate. Similarly then, in the absence of that 

 aetion of the medium by which was effected the differentiation of 



outer from inner in those microscopic portions of protoplasm consti- 

 tuting the earliest and simplest animals and plants, there could not 

 have existed this cardinal trait of composition which all the higher 

 animals and plants show us. 



So that, active as has been the part played by natural seleetion, 

 alike in modifying and moulding the original units largely as sur- 

 viva! of the fittest has been instrumental in farthering and controlling 

 the a cerebration of these units into visible organisms and eventually 

 into large ones : ye: we must ascribe to the direct effect of the me- 

 dium on primitive forms of life, that primordial trait of which this 

 everywhere-operative factor has taken advantage. 



Let us turn now : a another and more manifest trait of higher or- 

 nisms, for whieh also there is this same general cause. Let us ob- 



serve how, on a higher platform, there recurs this differentiation of 

 outer from inner how this primary trait in the living units with 

 whieh life eommenees, re-appears as a primary trait in those aggre- 

 - eh units whieh constitute visible organisms. 



I:: its simplest and most unmistakable form, we see this in the 

 early ehanges of an unfolding ovum of primitive type. The original 

 fertilized single cell, having by spontaneous fusion multiplied into a 

 cluster of sueh cells, there begins to show itself a contrast between 

 periphery and centre ; and presently there is formed a sphere consist- 

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

 is the rise of a difference between that outer part which holds direct 

 ".verse 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 bv its medium. 



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

 pha?nogam, we find, at all stages, a contrast between the inner and 

 outer parts of these flattened masses of tissue. In the higher Alg 

 "' he outermost lavers consist of smaller and firmer cells, while the 

 inner cells are often very large, and sometimes extremely long : n * 

 and in the leaves of trees the epidermal layer, besides differing in the 

 sizes and shapes of its component cells from the parenchyma forming 

 the inner substance of the leaf, is itself differentiated bv having a con- 



* Sachs, p. 210. 



