No doubt, as Mr.Wallace has remarked with much truth, a limit will be at last reached.

For instance, there must be a limit to the fleetness of any terrestrial animal, as this will be determined by the friction to be overcome, the weight of the body to be carried, and the power of contraction in the muscular fibres.

But what concerns us is that the domestic varieties of the same species differ from each other in almost every character, which man has attended to and selected, more than do the distinct species of the same genera.

Isidore Geoffroy St.Hilaire has proved this in regard to size, and so it is with colour, and probably with the length of hair.

With respect to fleetness, which depends on many bodily characters, Eclipse was far fleeter, and a dray-horse is comparably stronger, than any two natural species belonging to the same genus.