But what difference in the real internal constitution makes a specific difference it is in vain to inquire; whilst our measures of species be, as they are, only our abstract ideas, which we know; and not that internal constitution, which makes no part of them.

Shall the difference of hair only on the skin be a mark of a different internal specific constitution between a changeling and a drill, when they agree in shape, and want of reason and speech?
And shall not the want of reason and speech be a sign to us of different real constitutions and species between a changeling and a reasonable man?
And so of the rest, if we pretend that distinction of species or sorts is fixedly established by the real frame and secret constitutions of things.
23.

Species in Animals not distinguished by Generation.
Nor let any one say, that the power of propagation in animals by the mixture of male and female, and in plants by seeds, keeps the supposed real species distinct and entire, For, granting this to be true, it would help us in the distinction of the species of things no further than the tribes of animals and vegetables.

What must we do for the rest?
But in those too it is not sufficient: for if history lie not, women have conceived by drills; and what real species, by that measure, such a production will be in nature will be a new question: and we have reason to think this is not impossible, since mules and jumarts, the one from the mixture of an ass and a mare, the other from the mixture of a bull and a mare, are so frequent in the world.

I once saw a creature that was the issue of a cat and a rat, and had the plain marks of both about it; wherein nature appeared to have followed the pattern of neither sort alone, but to have jumbled them both together.