190 MISCELLANEOUS OBJECTIONS TO THE 



insists that the families of plants differ chiefly from each 

 other in morphological characters, which appear to be quite 

 unimportant for the welfare of the species. He conse- 

 quently believes in an innate tendency toward progressive 

 and more perfect development. He specifies the arrange- 

 ment 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, 

 etc. 



There is much force in the above objection. Neverthe- 

 less, 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 diminished flow of nutri- 

 ment to a part, mutual pressure, an early developed part 

 afi ecting 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 under- 

 stand. 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 sub- 

 ordinate 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 varia- 

 tions ; 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 differ- 

 ence, 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 im- 



