54 THE ORGANIC GROWTH OF THE LIVING WORLD SEC. 



especially with regard to colours and markings, only the 

 smallest part of which are published, show, however, in the 

 most distinct way, that the directions, not only of variation, 

 due, for example, to parasitism, but, as I may express it, of free 

 variation, in very many cases tend towards simplification and 

 not towards greater complexity. How far it is true in such 

 cases that the partial simplification is connected with simul- 

 taneous advance in evolution in another respect, and how 

 much is to be attributed to adaptation, must be ascertained 

 for each individual case ; in my examples these questions 

 have been considered. The facts already published by me on 

 the markings of animals, the transformation of longitudinal 

 striping into spots, then into transverse striping, and the final 

 disappearance of the marking, are sufficient to contradict 

 Nageli's principle of increased perfection. 



But my inquiries on the markings of butterflies will 

 furnish still more striking examples of the fact that variations 

 tending to simplicity as well as those to greater complexity 

 take place with wonderful persistence in direction, as if 

 following a path previously ordained. Such variation is 

 often in the same definite direction in species nearly related 

 but completely separated in habitat or even belonging to 

 quite different faunae, so that sometimes external conditions 

 appear to be undoubtedly the essential controlling factor. 

 Moreover, in many cases we have simplification alone, un- 

 compensated by greater complexity in another part. Thus 

 these examples also show once more that neither the inferior 

 role ascribed to adaptation by Nageli nor the commanding 

 one given by Weismann are justified. 



The process of evolution is not, as Nageli believes, to be 

 compared to an eternally growing tree, on which adaptation acts 

 only by cutting branches away and by pruning the whole. 

 Adaptation has also, if only indirectly, a perfecting and 

 strengthening in any case, a modifying effect. On the tree 



