268 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII 



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 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 vears 

 ago would have been considered as mere morphological 

 differences without any special function ; but they are 

 now known to be of the highest importance for the 

 fertilisation of the species through the aid of insects, 

 and have probably been gained through natural selec- 

 tion. No one until lately would have imagined that in 

 dimorphic and trimorphic plants the different lengths of 

 the stamens and pistils, and their arrangement, could have 

 been of any service, 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 

 phvsiological signification ; but Dr. Hooker informs me 

 that within the same ovarium, the upper ovules alone in 

 some cases, and in other cases the lower ones alone are 



