I pass on from that now, as I merely wished to explain to you what I
meant by the use of the term 'Geographical Distribution.' As I said,
there is another aspect, and a much more important one, and that is,
the relations of the various animals to one another. The Horse is a
very well-defined matter-of-fact sort of animal, and we are all pretty
familiar with its structure. I dare say it may have struck you, that
it resembles very much no other member of the animal kingdom, except
perhaps the Zebra or the Ass. But let me ask you to look along these
diagrams. Here is the skeleton of the Horse, and here the skeleton of
the Dog. You will notice that we have in the Horse a skull, a backbone
and ribs, shoulder-blades and haunch-bones. In the fore-limb, one
upper arm-bone, two fore arm-bones, wrist-bones (wrongly called knee),
and middle hand-bones, ending in the three bones of a finger, the last
of which is sheathed in the horny hoof of the fore-foot: in the
hind-limb, one thigh-bone, two leg-bones, anklebones, and middle
foot-bones, ending in the three bones of a toe, the last of which is
encased in the hoof of the hind-foot. Now turn to the Dog's skeleton.
We find identically the same bones, but more of them, there being more
toes in each foot, and hence more toe-bones.

Well, that is a very curious thing! The fact is that the Dog and the
Horse--when one gets a look at them without the outward impediments of
the skin--are found to be made in very much the same sort of fashion.
And if I were to make a transverse section of the Dog, I should find
the same organs that I have already shown you as forming parts of the
Horse. Well, here is another skeleton--that of a kind of Lemur--you
see he has just the same bones; and if I were to make a transverse
section of it, it would be just the same again. In your mind's eye
turn him round, so as to put his backbone in a position inclined
obliquely upwards and forwards, just as in the next three diagrams,