174 THE ORIGIN OF SPECIES 



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 devel- 

 oped 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 condi- 

 tions 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 in- 

 stances 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 vari- 

 ations 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 underrated, 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 under- 

 stood, excited so much surprise in the minds of the older nat- 

 uralists, can thus be explained. 



It may be worth while to illustrate some of the foregoing re- 

 marks. 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 be of no special use, I may mention that, 



