VIII] The Nature of Variation 147 



As the nature and properties of each of these units are 

 successively determined, we cannot doubt that additions will 

 be made to the number of examples, already not inconsider- 

 able, in which a fixed interrelation can be proved to exist 

 between the units which govern colour and those responsible 

 for form and other physiological attributes. 



In any attempt to picture the process of Evolution the 

 group of genetic phenomena discovered in regard to colour 

 has extreme value and interest. We thus are at once pro- 

 vided with clear illustrations which enable us to see the 

 nature, if not as yet the causation, of Variation, and the 

 significance of those particular Variations which we call 

 reversionary. Such illustrations may well serve as test- 

 cases, by which the truth of evolutionary systems may be 

 gauged. Though the result of these trials may largely prove 

 destructive, the facts are not without a constructive bearing. 

 One positive deduction cannot be overlooked : that the 

 organism is so built up that definite additions to, or sub- 

 tractions from its totality may readily be made by Variation, 

 and that the consequence of such alteration of the ingredients 

 may be recognizably definite, or to use another term, 

 specific 



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