THE LAW OF EVOLUTION 



which it has sprung ; and an orange is hetero- 

 geneous as compared with a wooden ball. 



III. Differentiation is the arising of an unlike- 

 ness between any two of the units which go to 

 make up an aggregate. It is the process through 

 which objects increase in heterogeneity. A piece 

 of cast-iron before it is exposed to the air is 

 relatively homogeneous. But when, by expo- 

 sure to the air, it has acquired a coating of ferric 

 oxide, or iron-rust, it is relatively heterogene- 

 ous. The units composing its outside are un- 

 like the units composing its inside ; or, in other 

 words, its outside is differentiated from its in- 

 side. 



IV. The term integration we have already 

 partly defined as the concentration of the ma- 

 terial units which go to make up any aggregate. 

 But a complete definition must recognize the 

 fact, that, along with the integration of wholes, 

 there goes on (in all cases in which structural 

 complexity is attained) an integration of parts. 

 This secondary integration may be defined as 

 the segregation, or grouping together, of those 

 units of a heterogeneous aggregate which resem- 

 ble one another. A good example is afforded 

 by crystallization. The particles of the crystal- 

 lizing substance, which resemble each other, 

 and which do not resemble the particles of the 

 solvent fluid, gradually unite to form the crys- 

 tal, which is thus said to be integrated from 



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