CHAPTER XVI. 



THE MODIFICATION OF GENERIC CHARACTERS, OR 

 GENERIC LIFE-HISTORY. 



In the last chapter the conclusion was reached that evo- 

 lution, which is the acquirement by organisms in the course 

 of individual growth of characters not previously appearing 

 in their ancestors, maybe distinguished as of two kinds: one 

 intrinsic, and expressing steps of progress in the differentia- 

 tion of function and organization of the organism as a whole, 

 working from within outward ; the second extrinsic in nature, 

 and expressed in the modification or adjustment of characters 

 already differentiated to local and immediate conditions of 

 environment. 



We observed that as the particular characters examined 

 are of higher and higher rank in classification they are more 

 intensely intrinsic in nature, not only now, but were so in the 

 earliest organisms of which we have any knowledge. And 

 still further, that these more essential characters were earlier 

 evolved, and the evidence seems to prove beyond doubt that 

 their evolution was by steps more rapid than would be in- 

 ferred from the relatively slow progress in the succession of 

 the lesser characters, generic and specific. 



Having noted the general laws of evolution respecting the 

 more important characters of each individual, we next turn 

 to an examination of the laws of evolution of the less im- 

 portant generic characters. 



In the generic characters there appears to have been a 

 rapid attainment of the total limit of modification expressed 

 anywhere in the family, with a long persistence of the more 

 widely divergent characters. When we examine the specific 

 and varietal characters we observe a much slower rate of 

 modification in individual race-series, but even here a re- 



276 



