628 H. S. 'Jennings 



10 It follows from what has been set forth in the paragraphs 

 preceding, that in the production of a new inherited character the 

 original modification will be something quite different from the 

 visible structural characteristic which later appears in consequence 

 of it. The original modification will be some chemical or struc- 

 tural change in the reproductive cell or cells that are later to pro- 

 duce the structure in question. (By producing in Paramecium 

 a localized change in the character of the protoplasm, a spine is 

 lat€r produced at that spot, etc.) The first appearance of the 

 visible structure is one generation after the production of the modifi- 

 cation to which it is due. 



11 Not all modifications of the germ cells that result in the 

 production of a new character in the next generation, will result 

 in the repeated production of this character in succeeding genera- 

 tions. In most cases, the new structure appears but once, and is 

 not inherited. In order that the new structure shall be inherited, 

 the original modification to which it is due must be transmitted 

 to the succeeding generation of germ cells. This is by no means 

 a matter of course; in fact, it is something not to he expected, as a 

 rule. The cell usually, by regulative processes, throws off after 

 a time any modification which the environment has impressed 

 upon it. Many examples of this are seen in the foregoing pages. 

 Certain unusual conditions of the cell result in the production, at 

 the next fission, of a spine. But during fission regulation occurs; 

 the unusual condition disappears, and the spine is not again pro- 

 duced. 



This is doubtless the fate of most modifications of the cell. We 

 saw, however, one modification which persisted, producing its 

 efi^ect in succeeding generations (pp. 618-622). Of such a nature 

 must be all modifications which produce new inherited character- 

 istics. It is easy to so modify the cell that new characteristics shall 

 appear in one succeeding generation; to so modify it that the new 

 characteristic shall appear regularly in succeeding generations is a 

 totally difi^erent matter. 



We often hear it pointed out that heredity is not transmission, 

 but new production; and this has been emphasized in the pre- 

 ceding pages. But it needs to be realized that while it is true that 



