THE FACTORS OF ORGANIC EVOLUTION. 193 



naked eye, are severally compounded of invisible organisms using 

 that word in its most comprehensive sense. And then, when the de- 

 velopment of any animal is traced, it is found that having been prima- 

 rily a nucleated cell, and having afterwards become by spontaneous 

 fission a cluster of nucleated cells, it goes on through successive stages 

 to form out of such cells, ever multiplying and modifying in various 

 ways, the several tissues and organs composing the adult. 



On the hypothesis of evolution this universal trait has to be ac- 

 cepted not as a fact that is strange but unmeaning. It has to be ac- 

 cepted as evidence that all the visible forms of life have arisen by 

 union of the invisible forms ; which, instead of flying apart, when 

 they divided, remained together. Various intermediate stages are 

 known. Among plants, those of the Volvox type show us the com- 

 ponent protophytes so feebly combined that they severally carry on 

 their lives with no appreciable subordination to the life of the group. 

 And among animals, a parallel relation between the lives of the units 

 and the life of the group is shown us in Uroglena and Syncrypta. 

 From these first stages upwards, may be traced through successively 

 higher types, an increasing subordination of the units to the aggre- 

 gate ; though still a subordination leaving to them conspicuous 

 amounts of individual activity. Joining which facts with the phe- 

 nomena presented by the cell-multiplication and aggregation of every 

 unfolding germ, naturalists are now 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, by analogy, I suppose will be called Metap>hyta, 

 though the word does not yet seem to have become current. 



And now what is the general meaning of these truths, taken in 

 connexion with the conclusion reached in the last section ? It is that 

 this universal trait of the Metazoa and Metaphyta, 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 proto- 

 plasm, 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 w T hich, w T hile being in some cases select- 

 ed, have been in all cases rounded and smoothed. Suppose a bed of 

 such shingle to be, as we often see it, solidified, along w T ith interfused 

 material, into a conglomerate. What in such case must be considered 

 as the chief trait of such conglomerate ; or rather what must we re- 

 gard as the chief cause of its distinctive characters ? Evidently the 

 action of the sea. Without the breakers, no pebbles ; without the 



* A Treatise on Comparative Embryology, By F..M. Balfour.. Tol. II, chap. xiii. 

 vol. xxix. 13 



