46 GERMINAL SELECTION. 



arrests the process, that is, ceases to eliminate th( 

 weaker individuals. 



In this manner it becomes intelligible how a larg< 

 number of modifications varying in kind and far mon 

 so in degree can be guided simultaneously by persona 

 selection; how in strict conformity with its adaptivt 

 wants every part is modified, or preserved unmodified 

 how a given articulation can undergo modifications 

 causing it to disappear on one side, to grow in volume 

 on another, and to continue unaltered on a third. Foi 

 every part that is perfectly adapted, although it car 

 fluctuate slightly, yet can never undergo a permanent 

 alteration in the ascending or descending direction 

 because every plus and every minus variation whicl 

 has attained selective value would be eliminated bj 

 personal selection in the course of time. Therefore, 

 a definite direction of variation cannot arise in such 

 cases and we have also reached, as it seems to me, 

 a satisfactory explanation of the constancy of well- 

 adapted species and characters. 



Hitherto I have spoken only of plus and minus varia- 

 tion. But there exist, as we know, not only variations 

 of size but also variations of kind; and the coloration 

 of the wings of butterflies, which we chose above as 

 our example, would fall, according to the ordinary 

 usage of speech, under just this head of variations 

 of quality. The question arises, therefore. Have the 

 principles just developed any claim to validity in the 

 explanation of qualitative modificati ons ? 



In considering this question it should be carefuUj 

 borne in mind that by far the largest part of the quali; 

 tative modifications" falliiig~uiider this head rest or 

 gMOMfi"^fj»g changes." Qf "course, chemical transforma- 

 tions, which usually also involve quantitative altera- 



