Chap. VII.] THEORY OF NATURAL SELECTION. 171 



There is much force in the above objection. Nevertheless, we 

 ought, in the first place, to he 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 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 oi 

 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 spon- 

 taneous 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 som6 

 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, which 

 before the principle of selection by man was well understood, excited 

 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 structures 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 



