As geology plainly proclaims that each land has undergone great physical changes, we might have expected to find that organic beings have varied under nature, in the same way as they have varied under domestication.

And if there has been any variability under nature, it would be an unaccountable fact if natural selection had not come into play.

It has often been asserted, but the assertion is incapable of proof, that the amount of variation under nature is a strictly limited quantity.