222 ORIGIN OF SPECIES 



A much more serious objection has been urged by Bronn, 

 and recently by Broca, namely, that many characters appear 

 to be of no service whatever to their possessors, and therefore 

 cannot have been influenced through natural selection. 

 Bronn adduces the length of the ears and tails in the dif- 

 ferent species of hares and mice, — the complex folds of 

 enamel in the teeth of many animals, and a multitude of 

 analogous cases. With respect to plants, this subject has 

 been discussed by Nageli in an admirable essay. He admits 

 that natural selection has effected much, but he insists that 

 the families of plants differ chiefly from each other in mor- 

 phological characters, which appear to be quite unimportant 

 for the welfare of the species. He consequently believes in 

 an innate tendency towards progressive and more perfect 

 development. He specifies the arrangement of the cells in 

 the tissues, and of the leaves on the axis, as cases in which 

 natural selection could not have acted. To these may be 

 added the numerical divisions in the parts of the flower, the 

 position of the ovules, the shape of the seed, when not of any 

 use for dissemination, &c. 



There is much force in the above objection. Nevertheless, 

 we ought, in the first place, to be extremely cautious in 

 pretending to decide what structures now are, or have for- 

 merly been, of use to each species. In the second place, it 

 should always be borne in mind that when one part is modi- 

 fied, so will be other parts, through certain dimly seen causes, 

 such as an increased or diminished flow of nutriment to a 

 part, mutual pressure, an early developed part affecting one 

 subsequently developed, and so forth, — as well as through 

 other causes which lead to the many mysterious cases of 

 correlation, which we do not in the least understand. These 

 agencies may be all grouped together, for the sake of brevity, 

 under the expression of the laws of growth. In the third 

 place, we have to allow for the direct and definite action of 

 changed conditions of life, and for so-called spontaneous 

 variations, in which the nature of the conditions apparently 

 plays a quite subordinate part. Bud-variations, such as the 

 appearance of a moss-rose on a common rose, or of a nec- 

 tarine on a peach-tree, offer good instances of spontaneous 

 variations; but even in these cases, if we bear in mind the 



