THEORY OF NATURAL SELECTION 223 



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 occasionally 

 arise; and if the unknown cause were to act persistently, it 

 is almost certain that all the individuals of the species would 

 be similarly modified. 



In the earlier editions of this work I under-rated, as it now 

 seems probable, the frequency and importance of modifica- 

 tions 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 selec- 

 tion 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 foregoing 

 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 use has not been, or has only 

 recently been, ascertained. As Bronn gives the length of 

 the ears and tail in the several species of mice as instances, 

 though trifling ones, of differences in structure which can 

 be of no special use, I may mention that, according to Dr. 

 Schobl, the external ears of the common mouse are supplied 

 in an extraordinary manner with nerves, so that they no 

 doubt serve as tactile organs ; hence the length of the ears 

 can hardly be quite unimportant. We shall, also, presently 

 see that the tail is a highly useful prehensile organ to some 

 of the species ; and its use would be much influenced by its 

 length. 



With respect to plants, to which on account of Nageli's 

 essay I shall confine myself in the following remarks, it will 

 be admitted that the flowers of orchids present a multitude of 

 curious structures, which a few years ago would have been 

 considered as mere morphological differences without any 



