46 GERMINAL SELECTION. 



arrests the process, that is, ceases to eliminate the 

 weaker individuals. 



In this manner it becomes intelligible how a large 

 number of modifications varying in kind and far more 

 so in degree can be guided simultaneously by personal 

 selection; how in strict conformity with its adaptive 

 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. For 

 every part that is perfectly adapted, although it can 

 fluctuate slightly, yet can never undergo a permanent 

 alteration in the ascending or descending direction 

 because every plus and every minus variation which 

 has attained selective value would be eliminated by 

 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 modifications? 



In considering this question it should be carefully 

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

 tative modifications falling under this head rest on 

 quantitative changes. Of course, chemical transforma- 

 tions, which usually also involve quantitative altera- 



