THEORY OF NATURAL SELECTION. 101 



possible to attribute to this cause the innumerable struc- 

 tures 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 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 struc- 

 tures 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. School, 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 unimpor- 

 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 Nageli'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 morphilogical differ- 

 ences without any special fuuction ; 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- 

 ably been gained through natural selection. No one until 

 lately would have imagined that in dimorphic and tri- 

 morphic plants the different lengths of the stamens and 

 pistils, and their arrangement, could have been of any ser- 

 vice, but now we know this to be the case. 



In certain whole groups of plants the ovules stand erect, 

 and in others they are suspended ; and within the same 

 ovarium of some few plants, one ovule holds the former 

 and a second ovule the latter position. These positions 

 seem at first purely morphological, or of no physiological 

 signification ; but Dr. Hooker informs me that within the 

 same ovarium, the upper ovules alone in some cases, and 



