468 



NA TURE 



{April 13, 1876 



If we had specimens of all known forms of crocodiles, 

 recent and extinct, and set to work to classify them 

 according to their degrees of likeness and unlikeness, we 

 should find that they naturally fell into three series. 



In the first of these it would be found that the skull 

 had all the characters mentioned at the end of the last 

 lecture, the posterior nares l^eing small apertures opening 

 into the cavity of the mouth behind the pterygoid bones ; 

 the vertebrte would be concave in front and convex be- 

 hind ; the two bones composing the shoulder-girdle, the 

 shoulder-blade or scapula and the coracoid, would be 

 similar in shape, both being long and narrow ; in the hip- 

 girdle, the haunch-bone or ilium would be much cut away 

 in front and excavated below, the ischium and pubis 

 being both long blade-like bones ; and there would be 

 seven or eight longitudinal rows of bony plates on the back. 



In the second set we should find the posterior nares to 

 be much larger and placed farther forwards, immediately 

 behind the palatine bones, the pterygoids not uniting as 

 in the first group. The vertebrae would be slightly hollowed 

 out at each end. In the shoulder and hip girdles there 

 would be no important difference from the first group, with 

 which also the more minute structure of the limbs would 

 correspond closely. A difierence would, however, be 

 found in the fact of there being not more than two rows of 

 plates on the back. 



In the third series, we should notice certain very 

 striking changes. The posterior nares would be actually 

 as far forward as in a lizard ; neither the palatine nor the 

 pterygoids uniting in the floor of the mouth ; the vertebrae 

 would be completely amphicoelous or biconcave ; the 

 coracoid no longer long and narrow, but expanded and 

 rounded like that of a lizard ; the ilium more elongated 

 and without the notch on its lower edge ; and the ischium 

 considerably broadened. As in the preceding group, the 

 rows of bony plates on the back would not exceed two. 



Thus we should find that the second group held an 

 exactly intermediate place between the first and third, and 

 that the third set, in every respect in which it differed 

 from the normal crocodilian structure, approached to that 

 of lizards. 



It is a very interesting point to see how these three 

 groups appear in time. We should find that in the first 

 are included all the Recent and Tertiary forms, and that 

 there are no indications of the type below the later 

 Cretaceous. 



The second group would be found to extend from the 

 older Cretaceous down to the Lias ; moreover, a careful 

 examination would show that there were lesser modifica- 

 tions among the individual species of a very instructive 

 nature ; those from the Wealden, for instance, would be 

 seen to have the posterior nares farther back {i.e., nearer 

 the typical crocodilian position) than those of the middle 

 Mesozoic, and these again than those of the Lias. 



The third group would contain exclusively Triassic 

 forms, such as the dragon-like Belodon and the Stag07io- 

 lepis of the Elgin sandstones. In this latter formation the 

 fossils are in a very curious condition ; after the sand 

 accumulated round the bodies of the Triasssic animals 

 had hardened, water, percolating through the porous rock, 

 completely dissolved out the bones, leaving nothing but 

 cavities. Thus we have only the remains of remains to 

 deal with, but casts taken from the cavities enable us to 

 make out, with perfect certainty, even important cha- 

 racters, although there may be hardly a bone left. 



We see then, that our third set of forms is the oldest, our 

 first the youngest, and the study of crocodilian remains 

 seem to show that that has happened in the history of 

 crocodiles, which should have happened, if the theory of 

 evolution be true. Anatomical characters show that 

 crocodiles are a modification of the lacertian type, and to 

 this type the Triassic species, from which we are cer- 

 tainly justified in supposing that existing forms are 

 descended, exhibit a marked approximation. 



Still we are very far from knowing the whole story : it 

 IS certainly allowable to assume that our third group of 

 crocodilian forms was evolved from a common stock with 

 lizards, but this is as far as the facts of the case will take 

 us at present. 



There seems, at first sight, to be something unnatural in 

 speaking of birds and reptiles together, for no two ani- 

 mals can be, to all appearance, more unlike. The won- 

 derfully constructed feathers of the one group, compared 

 with the scutes and scales of the other, the cold blood of 

 the reptile contrasted with the hot fluid which circulates 

 through the vessels of a bird and [raises its body several 

 degrees above our own in temperature ; the dumbness 

 and general sluggishness of the reptile as compared with 

 the vocal powers and the rapid flight of birds ; all these 

 compel us to say, and justly so, that nothing can be more 

 different than the character of the two classes. 



Even when we go more into details, similar differences 

 are apparent. The bird has a small head, set on a long 

 flexible neck, and provided with a horny beak in lieu of 

 teeth ; its bones are hollow and full of air ; its breast- 

 bone, instead of being a small plate of cartilage, is a 

 huge bony plate, usually provided with a large keel for 

 the attachment of the powerful muscles of flight ; the 

 fore-limb is of no use in progression on the ground, and, 

 the body having to be supported entirely by the hind 

 limbs, the femora are placed parallel to the long axis of the 

 body, instead of almost at right angles to it as in a reptile, 

 so that the body is well raised from the ground, and a 

 gait the very opposite of a reptile's sprawhng waddle is 

 the result. 



The scapula and coracoid are not so very different from 

 the corresponding bones in the lower class ; the humerus, 

 ulna, and radius, can also be perfectly well identified, but 

 the modification of the distal division of the limb — the 

 part answering to the reptile's fore-paw or to our own 

 hand — is very great. First come two small bones answer- 

 ing to carpals, then three longer ones all united together, 

 which represent the metacarpus, and are followed by the 

 rudiments of the phalanges of the three corresponding 

 digits. In the ostrich two of these three " fingers " are 

 terminated by claws, the use of which it is rather hard to 

 divine, unless the bird uses them for scratching itself, an 

 operation in which a very large portion of the activity of 

 the lower animals is taken up. 



The haunch-bone, or ilium, is of enormous size, and 

 extends a long way in front of, as well as behind, the 

 acetabulum ; in correspondence with this, a great number 

 of vertebrae are fused together to form a sacrum of suffi- 

 cient size for the attachment of the iha and the support of 

 the weight of the body. The ischium and pubis are long 

 slender bones, and the latter, as well as the former, is bent 

 back, so that they both come to lie nearly parallel with 

 the vertebral column. 



To allow of the femur taking up its position parallel with 

 the axis of the body, its well-finished globular head is set 

 on at right-angles to the shaft ; moreover, its further end 

 has a characteristic notch for the reception of the upper 

 extremity of the fibula. The shin-bone is provided with a 

 large and very characteristic crest for the attachment of 

 the strong muscles of the anterior part of the thigh ; its 

 lower extremity is pulley- shaped, and, in a young bird, the 

 pulley-like end, continued into a tongue of bone running 

 up the back of the tibia, can be separated as a per- 

 fectly distinct ossification ; its shaft also is so twisted that 

 its two ends come to lie in different planes. 



Following upon the tibia comes a bone with an easily 

 separable piece at its upper end, and showing signs of a 

 longitudinal division into three separate bones ; this is 

 the tarso-metatarsus, and represents the metatarsals and 

 all the tarsals except one — the astragalus— which is re- 

 presented by the pulley of the tibia. As a rule there are 

 four toes, three of which are turned forwards and articu- 

 late with the tarso-metatarsus, while the fourth, the repre- 



