thoughts in your eyes, and is as affectionate and obedient to his master
as it were to be wished all children were to their parents--this dog
is, as you must own, very superior, in all ways, to the frog, with its
large goggle eyes and clammy body, hiding itself in the water as soon
as you come near it. But again, the frog, which can come and go as it
likes, is decidedly superior to the oyster, which has neither head nor
limbs, and lives all alone, glued into a shell, in a sort of perpetual
imprisonment.

Now the machine I have been telling you about is found in the oyster
and in the frog as well as in the dog, only it is less complicated,
and therefore less perfect in the oyster than in the frog; and less
perfect again in the frog than in the dog; for as we descend in the
scale of animals we find it becoming less and less elaborate--losing
here one of its parts, there another, but nevertheless remaining still
the same machine to all intents and purposes; though by the time it
has reached its lowest condition of structure we should hardly be able
to recognize it again, if we had not watched it through all its
gradations of form, and escorted it, as it were, from stage to stage.

Let me make this clear to you by a comparison.

You know the lamp which is lit every evening on the drawing-room table,
and around which you all assemble to work or read. Take off first the
shade, which throws the light on your book--then the glass which
prevents it smoking--then the little chimney which holds the wick and
drives the air into the flame to make it burn brightly. Then take away
the screw, which sends the wick up and down; undo the pieces one by
one, until none remain but those absolutely necessary to having a light
at all--namely, the receptacle for the oil and the floating wick which