I would fain know why a shock and a hound are not as distinct species as a spaniel and an elephant.

We have no other idea of the different essence of an elephant and a spaniel, than we have of the different essence of a shock and a hound; all the essential difference, whereby we know and distinguish them one from another, consisting only in the different collection of simple ideas, to which we have given those different names.
39.

How Genera and Species are related to naming.
How much the making of species and genera is in order to general names; and how much general names are necessary, if not to the being, yet at least to the completing of a species, and making it pass for such, will appear, besides what has been said above concerning ice and water, in a very familiar example.

A silent and a striking watch are but one species, to those who have but one name for them: but he that has the name WATCH for one, and CLOCK for the other, and distinct complex ideas to which those names belong, to HIM they are different species.

It will be said perhaps, that the inward contrivance and constitution is different between these two, which the watchmaker has a clear idea of.
And yet it is plain they are but one species to him, when he has but one name for them.