[Reprinted from THE AMERICAN NATURALIST, Vol. XL1IJ., May, 11)09 ) 



THE CATEGORIES OF VARIATION 



PROFESSOR S. J. HOLMES 



UNIVERSITY OF T&iaeassstN <L/)t-tforNt A 



IT is a well-established fact that what are commonly 

 called variations include modifications of quite different 

 import in relation to the process of evolution. Whether 

 or not the variations that are induced in the soma, either 

 by its own activities or through the influences of the en- 

 vironment, have any effect in shaping the course of evolu- 

 tion as they were held to do by Lamarck and his followers, 

 it is evident that they do not count in this process in the 

 same way as variations that arise in the germ. But 

 among the germinal variations themselves there are 

 classes of unequal significance. Variations differ mark- 

 edly in regard to their stability or permanence. Many 

 variations after their first appearance persist with little 

 modification for an apparently indefinite time., Of these. 

 what are commonly called mutations afford conspicuous^ 

 examples; these are abrupt variations which breed true. 

 or nearly so from the start, having their own fluctuating 

 variability, to be sure, but around a mean which does not 

 approach that of the parental type in successive genera- 

 tions. Other variations behave quite differently. They 

 may be selected generation after generation, modifying 

 the stock up to a certain point, after which, if the variety 

 is left to itself, there is revision towards the original par- 

 ent. It is held by many "Chat these two classes of varia- 

 tions are fundamentally distinct, and that only the first, 



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