153 Heredity. 



change, if considerable, could hardly fail to affect certain 

 cells unfavorably; and it would therefore cause the pro- 

 duction of gemmules, thus inducing yariation in later 

 generations. 



Wc can also understand how a tendency to vary may 

 be hereditary, for if certain cells of the body vary, they 

 will exercise a disturbing effect upon adjacent or related 

 cells, and these, transmitting gemmules, will hand on the 

 tendency to vary to succeeding generations. 



Secondary Laius of Variation. 



The law that variability is itself hereditary involves 

 a number of secondary laws, all of which find a ready 

 explanation in our theory of heredity. 



Among these secondary laws is the law that '' sj^ecific 

 characters are more variable than generic characters." 

 Darwin has given the evidence of the existence of this 

 law (" Origin of Species," p. 122), so it will not be 

 necessary to discuss it, or to do more than point out that 

 the theory of heredity furnishes an explanation of it. 



The characters which are common to all the species 

 of a genus, and which distinguish it from other genera, 

 are, as a rule, much older than those which distiniruish. 

 one species of the genus from the other species. The 

 specific characters or features which distinguish each, 

 species of a genus from the others, are features which 

 have appeared as new variations since the time when the 

 various species diverged from the common ancestor from 

 whom they inherit their common or generic characters. 

 As specific characters are of more recent acquisition than, 

 generic characters, natural selection will have had less 

 time to act upon the former than upon the latter. The 

 adjustment between a specific character and its enviroa- 

 meut will therefore be, as a rule, less complete and per- 



