PROFESSOR HUXLEY'S VIEWS ON EVOLUTION 207 



thoroughly identified by the more or less complete skeletons, in a very perfect state of preservation, which he has 

 discovered. The Hesperornis, which measures between five and six feet in length, is astonishingly like our existing 

 divers or grebes in a great many respects ; so like them indeed that, had a skeleton of Hesperornis been found in a 

 museum without its skull, it probably would have been placed in the same group of birds as the divers and grebes 

 of the present day. But Hesperornis differs from all existing birds, and so far resembles reptiles, in one important 

 particular — it is provided with teeth. But the discovery of an animal which, in every part of its skeleton, closely 

 agrees mth existing birds, and yet possesses teeth, shows that there were ancient birds which, in respect of pos- 

 sessing teeth, approached reptiles more nearly than any existing bird does. The same formation has yielded 

 another bird, Ichthyornis, which also possesses teeth ; but the teeth are situated in distinct sockets, while those of 

 Hesferornis are not so lodged. The latter also has such very small, almost rudimentary wings, that it must have 

 been chiefly a swimmer and a diver like a penguin ; while Ichthyornis has strong wings, and no doubt possessed 

 corresponding powers of flight. The Solenhofen slates have yielded the Archvopteryx, the existence of which was 

 first made known by the finding of a fossil feather, or rather of the impression of one. By-and-by a solitary 

 skeleton was discovered, which is now in the British Museum. The feet are not only altogether bird-like, but have 

 the special characters of the feet of perching birds, while the body had a clothing of true feathers. Nevertheless, 

 in some other respects, Archivopteryx is unlike a bird and like a reptile. There is a long tail composed of many 

 vertebras. The structure of the wing difTers in some very remarkable respects from that which it presents in 

 a true bird. In the Archwopteryx, the upper-arm bone is like that of a bird, and the two bones of the fore- 

 arm are more or less like those of a bird, but the fingers are not bound together — they are free, and several, 

 if not all, of them were terminated by strong curved claws, not like such as are sometimes found in birds, but 

 such as reptiles possess ; so that, in the Archwopteryx, we have an animal which to a certain extent occupies a 

 midway place between a bird and a reptile. Like the Anoplof.herium and the Paln'otherium, therefore, Archxopteryx 

 tends to fill up the interval between groups which, in the existing world, are widely separated, and to destroy 

 the value of the definitions of zoological groups based upon our knowledge of existing forms. And such 

 cases as these constitute evidence in favour of evolution, in so far as they prove that, in former periods of the 

 world's history, there were animals which overstepped the bounds of existing groups, and tended to merge them 

 into larger assemblages." 



Eeferring to the horse, Mr. Huxley says : " It by no means follows, because the Palxotherium has much in 

 common with the horse, on the one hand, and with the rhinoceros on the other, that it is the intermediate 

 form through which rhinoceroses have passed to become horses, or vice versd ; on the contrary, any such supposition 

 would be certainly erroneous. Nor do I think it likely that the transition from the reptile to the bird has been 

 efTected by such a form as Archa'opteryx. And it is convenient to distinguish these intermediate forms between 

 two groups, which do not represent the actual passage from the one group to the other, as intercalary types, from 

 those linear types which, more or less approximately, indicate the nature of the steps by which the transition from 

 one group to the other was effected. 



" I conceive that such linear forms, constituting a series of natural gradations between the reptile and the bird, 

 and enabling us to understand the manner in which the reptilian has been metamorphosed into the bird type, 

 are really to be found among a group of ancient and extinct terrestrial reptiles known as the Ornithoscelida . The 

 remains of these animals occur throughout the series of Mesozoic formations, from the Trias to the Chalk, and there 

 are indications of their existence even in the later Palaeozoic strata. 



" Most of these reptiles, at present known, are of great size, some having attained a length of forty feet, or 

 perhaps more. The majority resembled lizards and crocodiles in their general form, and many of them were, hke 

 crocodiles, protected by an armour of heavy bony plates. But, in others, the hind-Umbs elongate and the fore- 

 limbs shorten, until their relative proportions approach those which are observed in the short-winged, flightless, 

 ostrich tribe among birds. 



" The skull is relatively Ught, and in some cases the jaws, though bearing teeth, are beak-like at their extremities, 

 and appear to have been enveloped in a horny sheath. In the part of the vertebral column which Ues between the 

 haunch bones and is called the sacrum, a number of vertebrae may unite together into one whole, and in this respect, 

 as in some details of its structure, the sacrum of these reptiles approaches that of birds. 



" But it is in the structure of the pelvis and of the hind limb that some of these ancient reptiles present the most 

 remarkable approximation to birds, and clearly indicate the way by which the most specialised and characteristic 

 features of the bird may have been evolved from the corresponding parts in the reptile. In the crocodile, the 

 pelvis is obviously composed of three bones on each side ; the ilium, the pubis, and the ischium. In the adult 

 bird there appears to be but one bone on each side. The examination of the pelvis of a chick, however, shows 

 that each half is made up of three bones, which answer to those which remain distinct throughout hfe in 



