Monstrous births.
Secondly, It would be necessary to know whether nature always attains that essence it designs in the production of things.

The irregular and monstrous births, that in divers sorts of animals have been observed, will always give us reason to doubt of one or both of these.
17.

Are monsters really a distinct species?
Thirdly, It ought to be determined whether those we call monsters be really a distinct species, according to the scholastic notion of the word species; since it is certain that everything that exists has its particular constitution.

And yet we find that some of these monstrous productions have few or none of those qualities which are supposed to result from, and accompany, the essence of that species from whence they derive their originals, and to which, by their descent, they seem to belong.
18.

Men can have no ideas of Real Essences.
Fourthly, The real essences of those things which we distinguish into species, and as so distinguished we name, ought to be known; i.e.we ought to have ideas of them.