THE LAW OF EVOLUTION 



a necessary consequence of the fundamental re- 

 lations of matter and motion. And the same is 

 true of that secondary integration or union of 

 like units, which serves to render differentiation 

 more conspicuous by substituting a demarcated 

 grouping for a vague one. Considering what 

 happens when a handful of pounded sugar, 

 scattered before the breeze, falls here and there 

 according to the respective sizes of the frag- 

 ments, — we perceive that the units which de- 

 scend in company are those of equal size, and 

 that their segregation results from their like re- 

 lations to the incident force. The integration of 

 several spinal vertebrae into a sacrum, as the 

 result of exposure to a continuous strain in the 

 same direction, is a still better example ; and 

 from the phenomena of morphological devel- 

 opment many parallel cases might be cited. 

 Wherever different parts of any group of units 

 stand in different relations to an incident force, 

 differentiation must result ; and wherever any 

 sub-group of these units, after becoming unlike 

 the rest, is acted on by a common force, the re- 

 sult must be the integration of the sub-group. 

 But manifestly the primary process of consol- 

 idation cannot long go on in any aggregate, 

 without bringing sundry groups of units into 

 dissimilar relations to adjacent groups ; nor can 

 it long go on without subjecting each group, 

 thus differentiated, to a predominant force ex- 

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