CHAP. VII.] THEORY OF NATURAL SELECTION. 159 



length of the ears and tails in the different 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 formerly been, of use 

 to each species. In the second place, it should always be borne 

 in mind that when one part is modified, so will be other parts, 

 through certain dimly seen causes, such as an increased or dimin- 

 ished flow of nutriment to a part, mutual pressure, an early deve- 

 loped 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 con- 

 ditions 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 nectarine on a peach-tree, offer good instances 

 of spontaneous variations ; but even in these cases, if we bear in 

 mind the power of a minute drop of poison in producing complex 

 galls, we ought not to feel too sure that the above 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 individual 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 modified. 



In the earlier editions of this work I under-rated, as it now 

 seems probable, the frequency and importance of modifications 

 due to spontaneous variability. But it is impossible to attribute 

 to this cause the innumerable structures 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, 



