Dinosaurs True Intermediate forms.

Such intermediate forms are to be found, however, by looking in a different direction. Through the whole series of mesozoic rocks there occur reptiles, some of which are of gigantic dimensions; in fact, they are reckoned among the largest of terrestrial animals. Some of them are forty and fifty, possibly more, feet long. Such are the iguanodon, the megalosaurus, and a number of others, with the names of which I will not trouble you. There are great diversities of structure among these great reptiles. Some of them resemble lizards in the proportions of their limbs, and have evidently walked on all fours, in such respect resembling the existing crocodile; but in others you can trace a series of modifications. The haunch-bone and what we call the appendages, the hind limbs, underwent a series of modifications, until at length they completely assumed the character of a bird's hind-limbs.

I here indicate (pointing to diagram) the hind-limb of a crocodile, showing the bones of the hind-limbs and of the pelvis. These are the haunch-bones; these are the other pelvic bones. Then comes the division of the foot which we call the tarsus, the bones of which are separate and distinct. Then come the four toes, which exist only in the hind-feet of the crocodile, and all of which are separate and distinct. The foot is flat on the ground, so that the legs spread out and the weight of the body hangs clumsily between them.

Contrast this with what we find in the bird. The haunch-bone here is immensely elongated and the joints of the back-bone between the two haunch-bones are united page 26 together so as to form a solid support, upon which the weight of the body rests. Then the thigh-bone becomes very short, and has a back ridge upon its outer articular surface. At the lower end the ridge fits in between the upper extremity of the small bone and the great bone—the fibula and tibia—and makes a kind of spring joint. Then this small bone of the leg is quite large above and becomes rudimentary below. It runs out into a style instead of being long and large, as it is in the case of the crocodile. Then, when you come to the bones of the foot you find there are no separate bones such as you have here, but the end of the tibia, the large bone of the leg appears to end in a kind of pulley, and that by a single bone supported upon all three toes. Upon the extremity of that bone are attached these three toes. It is obvious that the contrast between the crocodile's leg on the one hand, and the bird's leg on the other, is very striking. That gap or interval is completely filled up when you study the character of the hinder extremities in those ancient reptiles which are called the Dinosauria. I have here such a pelvis and such a hind leg. This bone in the crocodile is represented in the Dinosaur by that long bone which approaches in form to the corresponding bone of the bird. The thigh-bone of the Dinosaur lies parallel with the same bone as it does in the bird. In some of these birds all these four toes are turned forward, and they may be reduced to three; but these bones in the Dinosaur become so shaped that no motion is possible. Finding this modification in the limbs—in the Dinosaur the fore-limbs become smaller and smaller—the suspicion naturally arises that the animals may have assumed the erect position. That view was entertained by Mantel, and was also demonstrated to be probable by your own distinguished anatomist, Leidy; but the discoveries of late years show that in some of these forms it was actually so; that you had reptiles then that used their hind legs exactly as birds now do.

The diagram is a faithful and accurate representation of an existing fossil; except for this, that whereas in the existing fossil the bones are twisted about and out of place, I have put them here in the position that they must have had in Nature, and now you see a creature with a long neck and bird-like head, with very small anterior extremities, with a slender termination, which is in almost all respects like that of a bird, and that animal must assuredly have walked about upon its hind legs, bird fashion. Add to this creature feathers, and the transition would be complete for the other characters. The possession of teeth would, as we see, not separate the creature from the class of birds we have had. We have had to stretch the class of birds to birds having teeth, and so far as the character of the skeleton goes we may fairly say that there needs here little more than the addition of feathers—and whether this creature had them we don't know—to convert it into a bird.

I have said that there can be no question, from their anatomical structure, that these animals walked upon their hind legs, and in fact there are to be found in the page 27 strata of England gigantic footsteps, arranged in order like this of the Brontozovn, and which there can be no doubt were made by the Dinosaur, the remains of which were found in the same rock. And knowing that these reptiles that walked upon their hind legs and had the character generally of birds, did exist, it becomes a very important question whether those tracks in the Connecticut Valley, to which I referred just now, and which formerly used to be unhesitatingly referred to a class of birds, may not all have been made either by true reptiles of the Dinosaurian type, or whether, if we could get hold of the skeleton which made these tracks, some of which are marvellously like bird's tracks, we should not come upon exactly that series of transtitional forms by which in former days the reptile was connected with the bird.

I don't think, ladies and gentlemen, that I need insist upon the value of evidence of this kind. You will observe, that although it does not prove that birds have originated from reptiles by the gradual modification of the ordinary reptile into a Dinosaurian form, and so into a bird, yet it does show that that process may possibly have taken place, and it does show that there existed in former times creatures which filled up one of the largest gaps in existing animate Nature; and that was exactly the kind of evidence which I stated to you in starting we are bound to meet with in the rocks if the hypothesis of Evolution be correct.

In my next lecture, I will take up what I venture to call the demonstrative evidence of Evolution.