Lectures and Essays | Page 8

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, which represent the
skeletons of an Orang, a Chimpanzee, a Gorilla, and you find you have
no trouble in identifying the bones throughout; and lastly turn to the
end of the series, the diagram representing a man's skeleton, and still
you find no great structural feature essentially altered. There are the
same bones in the same relations. From the Horse we pass on and on,
with gradual steps, until we arrive at last at the highest known forms.
On the other hand, take the other line of diagrams, and pass from the
Horse downwards in the scale to this fish; and still, though the
modifications are vastly greater, the essential framework of the
organization remains unchanged. Here, for instance, is a Porpoise: here
is its strong backbone, with the cavity running through it, which
contains the spinal cord; here are the ribs, here the shoulder blade; here
is the little short upper-arm bone, here are the two forearm bones, the
wrist-bone, and the finger-bones.
Strange, is it not, that the Porpoise should have in this queer-looking
affair--its flapper (as it is called), the same fundamental elements as the
fore-leg of the Horse or the Dog, or the Ape or Man; and here you will
notice a very curious thing,--the hinder limbs are absent. Now, let us
make another jump. Let us go to the Codfish: here you see is the
forearm, in this large pectoral fin--carrying your mind's eye onward
from the flapper of the Porpoise. And here you have the hinder limbs

restored in the shape of these ventral fins. If I were to make a transverse
section of this, I should find just the same organs that we have before
noticed. So that, you see, there comes out this strange conclusion as the
result of our investigations, that the Horse, when examined and
compared with other animals, is found by no means to stand alone in
nature; but that there are an enormous number of other creatures which
have backbones, ribs, and legs, and other parts arranged in the same
general manner, and in all their formation exhibiting the same broad
peculiarities.
I am sure that you cannot have followed me even in this extremely
elementary exposition of the structural relations of animals, without
seeing what I have been driving at all through, which is, to show you
that, step by step, naturalists have come to the idea of a unity of plan, or
conformity of construction, among animals which appeared at first
sight to be extremely dissimilar.
And here you have evidence of such a unity of plan among all the
animals which have backbones, and which we technically call
"Vertebrata". But there are multitudes of other animals, such as crabs,
lobsters, spiders, and so on, which we term "Annulosa". In these I
could not point out to you the parts that correspond with those of the
Horse,--the backbone, for instance,--as they are constructed upon a
very different principle,