Chap. VII.] THEORY OP NATURAL SELECTION. ' 267 



third place, we have to allow for the direct and definite 

 action of changed conditions of life, and for so-called 

 spontaneous variations, in which the nature of the con- 

 ditions apparently plays a quite subordinate part. Bud- 

 variations, such as the appearance of a moss-rose on a 

 common rose, or of a nectarine on a peach-tree, ofEer 

 good instances of spontaneous variations; but even in 

 these eases, if we hear in mind the power of a minute 

 drop of poison in producing complex galls, we ought not 

 to feel too sure that the above variations are not the 

 effect of some local change in the nature of the sap, due 

 to some change in the conditions. There must be some 

 efficient cause for each slight individual difference, as 

 well as for more strongly marked variations which oc- 

 casionally arise; and if the unknown cause were to act 

 persistently, it Is almost certain that aU the individuals 

 of the species would be similarly modified. 



In th e^arlier editions of this work I under-rated, as 

 it now seems probable, the frequency and importance of 

 modifications due to spontaneous variability. But it is 

 impossible to attribute to this cause the innumerable 

 structures which are so well adapted to the habits of life 

 of each species. I can no more believe in this than 

 that the well-adapted form of a race-horse or greyhound, 

 which before the principle of selection by man was well 

 understood, excited so much surprise in the minds of 

 the older naturalists, can thus be explained. 



It may be worth while to illustrate some of the fore- 

 going remarks. With respect to the assumed inutility 

 of various parts and organs, it is hardly necessary to 

 observe that even in the higher and best-known animals 

 many structures exist, which are so highly developed 

 that no one doubts that they are of importance, yet their 



