THEORY OF NATURAL SELECTION. 203 



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 indi- 

 vidual 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 moditied. 



In the earlier editions of this work I underrated, as it 

 now seems probable, the frequency and importance of 

 modifications due to spontaneous variability. But it is im- 

 possible to attribute to this cause the innumerable struct- 

 ures 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, ex- 

 cited so much surprise in the minds of the older natural- 

 ists, 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 struct- 

 ures 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 unim})or- 

 tant. 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 Niigeli's 

 essay I shall confine myself in the following remarks, it 

 will be admitted that the flowers of the orchids present a 

 multitude of curious structures, which a few years ago 

 would have been considered as mere morphological differ- 

 ences without any special function; but they are now 

 known to be of the highest importance for the fertilization 

 of the species through the aid of insects, and have prob- 



