172 MISCELLANEOUS OBJECTIONS TO THE [CiiAr. VII. 



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

 the external ears of the common mouse are supplied in an extra- 

 ordinary manner with nerves, so that they no doubt serve as tactile 

 organs ; hence the length of the ears can hardly be quite unim- 

 portant. Wo 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. 



Wilh 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 morpho- 

 logical 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 selection. 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 physiological 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 fertilised ; and he 

 suggests that this probably depends on the direction in which the 

 pollen-tubes enter the ovarium. If so, the position of the ovules, 

 even when one is erect and the other suspended within the same 

 ovarium, would follow from the selection of any slight deviations in 

 position which favoured their fertilisation, and the production of seed. 



Several plants belonging to distinct orders habitually produce 

 flowers of two kinds, — the one open of the ordinary structure, the 

 other closed and imperfect. These two kinds of flowers sometimes 

 differ wonderfully in structure, yet may be seen to graduate into 

 each other on the same plant. The ordinary and open flowers can 

 be intercrossed ; and the benefits which certainly are derived from 

 this process are thus secured. The closed and imperfect flowers are, 

 however, manifestly of high importance, as they yield with the 

 utmost safety a large stock of seed, with the expenditure of wonder- 

 fully little pollen. The two kinds of flowers often differ much, as 

 just stated, in structure. The petals in the imperfect flowers almost 

 always consist of mere rudiments, and the pollen-grains are reduced 

 in diameter. In Ononis columnar five of the alternate stamens aro 



