76 CHARLES DARWIN. 



their own organs has been often refuted, but the view 

 here proposed Depends upon the action of principles 

 constantly working in nature. Retractile talons of 

 falcons and cats have not been developed by volition, 

 but by the survival of those which had the greatest 

 facilities for seizing prey. The long neck of the 

 giraffe was not produced by constant stretching, but 

 by the success which any increase in the length of 

 neck ensured to its possessors. Even colours, especi- 

 ally of insects, are explained in the same way, for 

 among the varieties of many tints, those "having 

 colours best adapted to concealment . . . would in- 

 evitably survive the longest." We can similarly 

 explain deficiency of some organs with compensating 

 development of others, " great velocity making up for 

 the absence of defensive weapons," etc. Varieties 

 with an unbalanced deficiency could not long survive. 

 The action of the principle is like the governor of a 

 steam-engine, checking irregularities almost before 

 they become evident. Such a view accords well with 

 " the many lines of divergence from a central type " ; 

 the increasing efficiency of a particular organ in a 

 series of allied species ; the persistence of unimportant 

 parts when important ones have changed ; the " more 

 specialised structure," said by Owen to be character- 

 istic of recent as compared with extinct forms. 



Hence there is a tendency of certain classes of 

 varieties to progress further and further from the 

 original type, and there is no reason for. assigning any 

 limit to this progression. Such gradual changes 



