But on the view that groups of species are descended from some other species, and have been modified through natural selection, I think we can obtain some light.

First let me make some preliminary remarks.

If, in our domestic animals, any part or the whole animal be neglected, and no selection be applied, that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a uniform character: and the breed may be said to be degenerating.