THE LAW OF EVOLUTION. 321 



110. Already we have recognized the fact that or- 

 ganic evolution is primarily the formation of an aggre- 

 gate, by the continued incorporation of matter previously 

 spread through a wider space. Merely reminding the 

 reader that every plant grows by concentrating in itself 

 elements that were before diffused as gases, and that every 

 animal grows by re-concentrating these elements previously 

 dispersed in surrounding plants and animals; it will be 

 here proper to complete the conception by pointing out that 

 the early history of a plant or animal, still more clearly than 

 its later history, shows us this fundamental process. For the 

 microscopic germ of each organism undergoes, for a long 

 time, no other change than that implied by absorption of 

 nutriment. Cells imbedded in the stroma of an ovarium, 

 become ova by little else than continued growth at the 

 expense of adjacent materials. And when, after fertiliza- 

 tion, a more active evolution commences, its most conspicu- 

 ous trait is the drawing-in, to a germinal centre, of the sub- 

 stance which the ovum contains. 



Here, however, our attention must be directed mainly to 

 the secondary integrations which habitually acompany the 

 primary integration. We have to observe how, along with 

 the formation of a larger mass of matter, there goes on a 

 drawing together and consolidation of the matter into 

 parts, as well as an increasingly-intimate combination of 

 parts. In the mammalian embryo, the heart, at, 



first a long pulsating blood-vessel, by and by twists upon 

 itself and integrates. The bile-cells constituting the rudi- 

 mentary liver, do not simply become different from the wall 

 of the intestine in which they at first lie; but, as they accu- 

 mulate, they simultaneously diverge from it and consolidate 

 into an organ. The anterior segments of the cerebro-spinal 

 axis, which are at first continuous with the rest, and distin- 

 guished only by their larger size, undergo a gradual union ; 

 and at the same time the resulting head folds into a mass 

 clearly marked off from the rest of the vertebral column. 



