200 
DIFFICULTIES ON THEOEY. 
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- 
