180 ON THE ORIGIN OF SPECIES 



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 nipper, 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 selec- 

 tion, subjected formerly, as now, to the several laws 

 of inheritance, reversion, correlation of growth, etc. 

 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 modi- 

 fication in any one species exclusively for the good of 

 another species ; though throughout nature one species 

 incessantly takes advantage of, and profits by, the 

 structure 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 deposited in the living bodies of other insects. If 

 it could be proved that any part of the structure of any 

 one species had been formed for the exclusive good, of 

 another species, it would annihilate my theory, for 

 such could not have been produced through natural 

 selection. Although many statements may be found 

 in works on natural history to this effect, I cannot find 

 even one which seems to me of any weight. It is 

 admitted that the rattlesnake has a poison-fang for its 



