ch. iv.] THE LAW OF EVOLUTION. 337 



also seen in the rising of cream upon the surface of a dish 

 of milk, and in the frothy collection of carbonic-acid bubbles 

 covering a newly-filled glass of ale. 



Obviously as it is through differentiation that an aggregate 

 increases in heterogeneity, so it is through integration that an 

 aggregate increases in definiteness, of structure and function. 

 But there is still another way in which integration is exem- 

 plified. Along with increasing heterogeneity and definiteness 

 of structure and function, the evolution of an aggregate i? 

 marked by the increasing subordination of the various func- 

 tions, with their structures, to the requirements of the general 

 functional activity of the aggregate. In other words, along 

 with growing specialization of parts, there is a growing 

 cooperation of parts, and an ever-increasing mutual de- 

 pendence among parts. An illustration is furnished by the 

 contrasted facts, that a slightly-evolved animal, like a 

 common earth-worm, may be cut in two without destroy- 

 ing the life of either part ; while a highly-evolved animal, 

 like a dog, is destroyed if a single artery is severed, or if 

 any one of the viscera is prevented from discharging its 

 peculiar functions. This third kind of integration is the 

 process through which an evolving aggregate increases in 

 coherence. And with this, our definition of the factors which 

 concur in the process of evolution is complete. 



We are now prepared to show inductively that wherever, 

 as in organic aggregates, the conditions permit, the integration 

 of matter and concomitant dissipation of motion, which 

 'primarily constitutes Evolution, is attended by a continuous 

 change from indefinite, incoherent homogeneity to definite, 

 coherent heterogeneity of structure and function, through 

 successive differentiations and integrations. In illustration of 

 this statement, let us describe first, some of the differentia- 

 tions, and secondly, some of the integrations, which suc- 

 cessively occur during the development of an individual 

 organism. 



VOL. L Z 



