ACQUIRED CHARACTERS IN ANIMAL BREEDING 481 



modifications of bodily structure or habit which are impressed upon 

 the organism in the course of individual life. This distinction is by 

 no means another instance of the hair-splitting proclivities of modern 

 science; it is on the contrary a real distinction of fundamental impor- 

 tance in shaping conceptions of evolution and heredity. It is un- 

 necessary to give any specific examples of blastogenic characters, since 

 the whole discussion of Mendelian heredity in preceding pages has been 

 confined to them. Of somatogenic characters, however, it is perhaps 

 well to mention a few in order to give a concrete starting point for the 

 following discussion. Acquired characters include a vast number of 

 characters due to environmental effects, for example, small size when 

 a consequence of reduced food supply or other conditions unfavorable 

 to growth, increased size consequent upon unusually favorable environ- 

 mental conditions, mutilation, the effects of disease, and other modifi- 

 cations of a like character. Those acquired characters which have 

 their origin in response to environmental conditions have often been 

 distinguished from that other class, the motive force in the develop- 

 ment of which resides in the organism itself, the effects of use and 

 disuse. Conspicuous examples of "achieved" characters as distin- 

 guished from "thrust" characters are increases in the perfection of 

 function dependent upon exercise, such as the increased speed of the 

 trained race horse and the increased sharpness of intellect of the 

 trained mind. 



As Thomson has stated it, the precise question at issue is this : Can 

 a structural change in the body, induced by some change in use 

 or disuse, or by a change in surrounding influence, affect the germ cells 

 in such a specific or representative way that the offspring will through its 

 inheritance exhibit, even in a slight degree, the modification which the 

 parent acquired? 



Obviously a problem such as this must require very critical treat- 

 ment, and much, if not all of the evidence brought forward in support 

 of the belief in the inheritance of acquired characters suffers from failure 

 to fulfil the requirements of a rigid proof. Thomson has given an excel- 

 lent extended treatment of this side of the case, as well as of the subject 

 of acquired characters in general. 



To satisfy the rigid requirements of an experimental proof any evi- 

 dence of the inheritance of acquired characters must fulfil the following 

 conditions : 



First, a specific character or modification in the soma must be im- 

 pressed upon the organism by a known factor in its environment or in 

 its exercise of bodily function. 



Second, the character or modification should be new. There must be 

 no question of the reappearance of ancestral traits or characters, or of 



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