HEREDITY 181 



rudiments were created in accordance with the tendency 

 in creative processes to adhere to an ideal type. But it can- 

 not be too clearly understood that tendencies in biology exist 

 only as functions of particular organs. The tendency to 

 adhere to a type is a part of heredity, the function of the germ 

 cell. 



In the light of our knowledge of organic evolution it is 

 clear that the presence of vestigial organs is simply a fact of 

 heredity. They are organs once useful, but which through 

 changed conditions of life have become needless. 



It is a recognized fact that useless organs tend to dwindle 

 away, but the cause of this phenomenon is not so clear. It may 

 be due in part to (a) panmixia or cessation of selection, the 

 organ being no longer held to a high grade of efficiency, to (b) 

 reversal of selection, the advantage lying with those individuals 

 in which the organ is no longer functional or (c) the inheritance 

 of the results of functional disuse. The latter offers an explana- 

 tion which at first sight appears adequate, and its reality has 

 been stoutly maintained by various writers of the Neo-Lamarck- 

 ian school. In their views, changes in the individuals unques- 

 tionably due to individual or ontogenetic disuse are carried over 

 to the species as phylogenetic disuse. Against this view is 

 opposed its inconsistence with current theories of heredity, and 

 also the positive fact that there is as yet no proof of the in- 

 heritance of acquired characters. 



When we say that, through heredity, the offspring inherits 

 the characters of the parent, we are speaking only a large and 

 general truth. The details of this inheritance reveal in what 

 regards this general statement must be modified. We have 

 already noted the inevitable occurrence of at least small varia- 

 tions in all body parts in all individuals. In addition to this ex- 

 ception to identical inheritance, certain characters of the parent 

 may not, as just mentioned, appear at all in the offspring. And 

 this may be due to any one of several causes. 



First, certain parental characters are apparently really not 

 heritable, namely, those new characters which have been ac- 

 quired by the parent during its lifetime as the result of mutila- 

 tion, disease, special use or disuse of parts, any change of parts 

 due to direct reaction to a functional stimulus or to an environ- 

 mental stimulus or cause, such as a bleaching due to lack 

 of light, a thickening of the skin in certain places due to con- 

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