PART II CAUSES OF VARIATION 



INTRODUCTION 



Variation is at once the most promising agent for improve- 

 ment and the most powerful and subtle force for undermining 

 and destroying what has already been attained. Because of this 

 and with a view to their possible control, the breeder is especially 

 interested in the causes that lead to deviation in plant or animal 

 characters. 



It is said that it is yet too early to inquire into the causes of 

 variation, because our stock of accurate knowledge is too limited 

 to permit a settlement of this most complicated question. That 

 .the matter cannot be fully settled in the present state of knowl- 

 edge is beyond question, but the writer does not share the opin- 

 ion that discussion at this stage of proceedings is unprofitable. 



The student of general evolution may well assume the role of 

 a curious but disinterested observer, note what passes before 

 his eyes, and take his choice as to the questions that shall 

 engage his attention. Not so with the farmer and breeder. His 

 funds are tied up in his animals and his plants. He is breeding 

 them not for amusement but for profit, and he is interested in 

 results not thousands of years hence but in those that may be 

 confidently expected within the limits of a lifetime. 



He above all men, therefore, is interested in variation and 

 the causes that induce it ; and we are bound in his interest to 

 study the question assiduously, to determine what is known 

 and what is not known on this most important point, and to 

 indicate as well as we are able the direction from which further 

 light may be expected. To this end everything is important 

 that is connected with variability in a causative way, whether 

 its effect is upon either the form or the function of living 

 matter. 



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