122 THE BEGINNINGS OF LIFE. 



aggregate prevents easy dissipation of the motion ; or 

 because though motion is rapidly lost more motion is 

 rapidly received, then other forces will cause in the 

 aggregate appreciable modifications. Along with the 

 change constituting integration there will take place 

 supplementary changes. The Evolution instead of 

 being simple will be compound ^' 



What usually occurs in the latter case is indicated 

 by the following definition 2 : — « Evolution is an inte- 

 gration of matter and concomitant dissipation of 

 motion; during which the matter passes from an 

 indefinite incoherent homogeneity, to a definite cohe- 

 rent heterogeneity; and during which the retained 

 motion undergoes a parallel transformation.' 



The presence of much retained internal motion in 

 an aggregate undergoing condensation, is the pecu- 

 liarity that, above all others, favours the occurrence 

 of the internal changes and re-arrangements which 

 characterize compound evolution. Yet whilst matter 

 still exists in those states in which it contains the 

 largest amount of this retained motion — whilst it is 

 still in the gaseous or liquid stages — the secondary 

 redistributions which undoubtedly take place vanish 

 almost as soon as they occur — ^ the molecular mobility 

 being such as to negative the fixed arrangement of 

 parts we call structure 3.' But on approaching solidity 

 ^we arrive at a condition called plastic, in which 



1 'First Principles,' 2nd ed. p. 287. 2 Lq(,_ ^^^^^ ^ 296. 



2 Loc. cit., p. 297. 



