CHAPTER XI 



TRANSMISSION OF MODIFICATIONS DUE TO EXTERNAL 

 INFLUENCES 



SECTION I INTRODUCTORY 



In the discussion of the causes of variation a clear distinction 

 was made between causes internal and causes external to the 

 organism. This distinction should still be borne in mind, though 

 the principal discussion and all of the controversy arise in con- 

 nection with external causes. 



Variations due to causes internal to the germ are transmitted. 

 So far as the writer is aware, no one has ever disputed, or even 

 questioned, the correctness of this proposition. It is fundamental, 

 if not axiomatic, that any change taking place in the structure 

 of the germinal matter, which is passed on from generation 

 to generation, which is the bearer of all characters and the 

 exclusive basis of all transmission, that any such alteration 

 will of necessity make itself manifest at once, and indefinitely 

 afterward ; indeed, as long as the change in the constitution of 

 the germ continues. This is the purpose of all selective mating, 

 to secure a germ endowed with as many as possible of what 

 are considered to be desirable variations, tending to maximum 

 development, and as few as possible that are undesirable, and 

 these with a minimum development. Such a germ should 

 develop an embryo, and finally an adult individual, with the 

 highest obtainable degree of excellence. 



To control the germ is the purpose of all selection. 'It is the 

 object of all breeding. We practice line breeding, and even 

 in-breeding, to give intensity along certain lines ; we resort to 

 mixed breeding, even to cross breeding, to introduce new vari- 

 ations and to endow the race with greater flexibility. 



It is in the germ that mutations arise. The causes are 

 obscure, but it is here, in the constitution of the germinal 



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