200 DIFFICULTIES ON THEORY. Chap. VI. 



goose or of the frigate-bird are of special use to these 

 birds ; we cannot believe that the same bones in the 

 arm of the monkey, in the fore leg of the horse, in the 

 wing of the bat, and in the flipper of the seal, are of 

 special use to these animals. We may safely attribute 

 these structures to inheritance. But to the progenitor 

 of the upland goose and of the frigate-bird, webbed feet 

 no doubt were as useful as they now are to the most 

 aquatic of existing birds. So we may believe that the 

 progenitor of the seal had not a flipper, but a foot with 

 five toes fitted for walking or grasping ; and we may 

 further venture to believe that the several bones in the 

 limbs of the monkey, horse, and bat, which have been 

 inherited from a common progenitor, were formerly of 

 more special use to that progenitor, or its progenitors, 

 than they now are to these animals having such widely 

 diversified habits. Therefore we may infer that these 

 several bones might have been acquired through 

 natural selection, subjected formerly, as now, to the 

 several laws of inheritance, reversion, correlation of 

 growth, &c. Hence every detail of structure in every 

 living creature (making some little allowance for the 

 direct action of physical conditions) may be viewed, 

 either as having been of special use to some ancestral 

 form, or as being now of special use to the descendants 

 of this form — either directly, or indirectly through the 

 complex laws of growth. 



Natural selection cannot possibly produce any modifi- 

 cation in any one species exclusively for the good of 

 another species ; though throughout nature one species 

 incessantly takes advantage of, and profits by, the struc- 

 ture of another. But natural selection can and does 

 often produce structures for the direct injury of other 

 species, as we see in the fang of the adder, and in the 

 ovipositor of the ichneumon, by which its eggs are depo- 



