CHAPTER IV 



THE CLASSIFICATION OF VARIATION AND THE NATURE OF 

 SUBSTANTIVE FACTORS 



We have now seen that among the normal physiological 

 processes the phenomena of division form a recognisable, and 

 in all likelihood a naturally distinct group. Variations in these 

 respects may thus be regarded as constituting a special class 

 among variations in general. 



The substantive variations have only one property in com- 

 mon the negative one that they are not Meristic. The work 

 of classifying them and distinguishing them according to their 

 several types demands a knowledge of the chemistry of life far 

 higher than that to which science has yet attained. In reference 

 to some of the simplest variations Garrod has introduced the 

 appropriate term "Chemical sports." The condition in man 

 known as Alkaptonuria in which the urine is red is due especially 

 to the absence of the enzyme which decomposes the excretory 

 substance, alkapton. The " chemical sport" here consists in the 

 inability to break up the benzene ring. The chemical feature 

 which distinguishes and is the proximate cause of several colour- 

 varieties can now in a few cases be declared. The work of Miss 

 Wheldale has shown that colour-varieties may be produced by 

 the absence of the chromogen compound the oxidation of which 

 gives rise to sap-colours, by differences in the completeness of 

 this process of oxidation, and by a process of reduction super- 

 vening on or perhaps suppressing the oxidation. Some of these 

 processes moreover may be brought about by the combined action 

 of two bodies, the one an enzyme, for example an oxygenase, and 

 the other a substance regarded as a peroxide, contributing the 

 oxygen necessary for the oxidation to take place. Variation in 

 colour may thus be brought about by the addition or omission 

 of any one of the bodies concerned in the action. 



Similar variations, or rather similar series of variations will 



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