Common-sense biology reasons as follows. In a living body there is a
certain pattern of organized parts, a certain rhythm of successive motions,
and a certain range of characteristic activities. The pattern, the sheer
anatomy, is basic; but it cannot long continue to exist (outside a
refrigerator) without accompanying vital rhythms in heart, respiration and
digestion. Nor do these perform their parts without the intermittent
support of variable but still characteristic activities: dogs not only
breathe and digest, they run about, hunt their food, look for mates, bark
at cats, and so on. The anatomical pattern, the vital rhythm, and the
characteristic acts together express dogginess; they reveal the specific
form of the dog. They _reveal_ it; exactly what the specific form
_consisted in_ was the subject of much medieval speculation. It need not
concern us here.

Taking the form of the species for granted, common-sense biology proceeds
to ask how it comes to be in a given instance, say in the dog Toby. [15]
Before this dog was born or thought of, his form or species was displayed
in each of his parents. And now it looks as though the form of dog had
detached itself from them through the generative act, and set up anew on
its own account. How does it do that? By getting hold of some materials in
which to express itself. At first it takes them from the body of the
mother, afterwards it collects them from a wider environment, and what the
dog eats becomes the dog.

What, then, is the relation of the assimilated materials to the dog-form
which assimilates them? Before assimilation, they have their own form.
Before the dog eats the leg of mutton, it has the form given to it by its
place in the body of a sheep. What happens to the mutton? Is it without
remainder transubstantiated from sheep into dog? It loses all its