5G THE FACTORS OP ORGANIC EVOLUTION. 



accepting the conclusion that by this process of composition 

 from Protozoa, were formed all classes of the Metazoa* (as 

 animals formed by this compounding are now called) ; and 

 that in a similar way from Protophyta,were formed all classes 

 of what I suppose will be called Metaphyta, though the 

 &quot; word does not yet seem to have become current. 



And now what is the general meaning of these truths, 

 taken in connexion with tin conclusion reached in the 

 last section. It is that this universal trait of the Metazoa 

 and Metapkyta, must be ascribed to the primitive action 

 and re-action between the organism and its medium. The 

 operation of those forces which produced the primary 

 differentiation of outer from inner in early minute masses 

 of protoplasm, pre-determined this universal cell-structure 

 of all embryos, plant and animal, and the consequent cell- 

 composition of adult forms arising from them. How 

 unavoidable is this implication, will be seen on carrying 

 further an illustration already used that of the shingle- 

 covered shore, the pebbles on which, while being in some 

 cases selected, have been in all cases rounded and smoothed. 

 Suppose a bed of such shingle to be, as we often see 

 it, solidified, along with interfused material, into a con 

 glomerate. What in such case must be considered as the 

 chief trait of such conglomerate; or rather what must we 

 regard as the chief cause of its distinctive characters ? 

 Evidently the action of the sea. Without the breakers, no 

 pebbles ; without the pebbles, no conglomerate. Similarly 

 then, in the absence of that action of the medium by which 

 was effected the differentiation of outer from inner in those 

 microscopic portions of protoplasm constituting 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 

 selection, alike in modifying and moulding the original 

 * A Treatise on Comparative Embryology, by F. M. Balfour, Vol. ii, chap. xiii. 



