82 



NATURE 



\May 2 2, 1879 



The influence of desert or prairie life has doubtless 

 had much to do with the modifications found in the 

 ostrich tribe ; the principal of these modifications are, the 

 abortion of the tail, the lessening of the wing, the 

 huge increase in the size of the hind-limbs, and the 

 suppression of unnecessary toes. 



It is quite within the bounds of reasonable conjecture 

 to suppose that from time to time great waves of morpho- 

 logical force, so to speak, and fresh revivals of life, 

 passed over the old reptilian fauna, and that many kinds 

 of the more ductile reptiles yielded to the influence of 

 these waves. When, however, one member improved, the 

 other members improved with it ; the loss of digits and 

 their claws, the sport that appeared in the exchange of 

 feathers for scales, the increased solidity of the sacral 

 region, and the tighter setting on of the hind limbs — these 

 were only a few of the things that were correlated in this 

 radical reform of the old reptilian types. The teeth were 

 only slowly lost, as the invaluable papers of Prof. Marsh 

 show. Epidermal imitations of teeth, however, occur in 

 existing ducks and geese, and in the South American 

 Passerine, Phyiotoma rara, in which latter they are 

 ossified from the jaw. 



The hot condition of the bird's blood has much to do 

 with the intensity and rapidity of its early metamorphosis. 

 The time in which arboreal birds {Allrices) are ripened 

 for hatching is marvellously short, and after this, in a 

 single month, many of them have learnt their lesson in 

 flying, and begin to be ready to migrate. 



The segmented form of the simpler fish, or amphibian, 

 is still to be traced in the bird. The embryo of the swan 

 develops sjx dpzen segments behind the head, twenty- 

 five of which belong to the neck. Most of these segments 

 can be traced in the adult, except those of the tail, the 

 posterior ten of which, though separate at the time of 

 hatching, grow together to form the plough-share bone, 

 on which the tail-quills are set. 



The endo-skeleton is greatly developed, and its ossifi- 

 cation is carried to the utmost degree of perfection, only 

 traces of cartilage persisting in the joints and often in 

 the nose. 



The outer skeleton may be divided into three categories. 

 The first of these consists of the exuberant and unique 

 growths of the epidermis — the feathers and quills, to 

 which must be added the horny sheaths of the beak and 

 the scaly coverings of the legs. The second consists of 

 the overlapping series of cartilages that become ossified 

 to form the limb-girdles from which the limbs grow out, 

 and also a pair of sub-cutaneous cartilages in front of the 

 head, which belong to the labial category, and sometimes 

 there is one on each ramus of the mandible. The third 

 takes in the sub-cutaneous bones that invest the proper 

 endo-skeleton, whether bony or cartilaginous, and the 

 ossified tendons and fasciae {apo are ureses). There are 

 three proper investing bones, as a rule, attached to the 

 shoulder-girdle, which coalesce to form the greater part 

 of the merrythought (Jurculd); these, as before men- 

 tioned, are the counterparts of the ventral splint-bones of 

 Plesiosaurus and Hatteria, and their non-coalesced repre- 

 sentatives are also seen in lizards and in the Mono- 

 tremes. In the head there is a large number of investing 

 bones, which, however, enjoy for the most part a very 

 temporary independent existence. 



The increased size of the brain-mass has given rise to 

 a very different proportion of cranial to facial elements 

 as compared with what exists in the reptile, and the pro- 

 cess of ossification is carried to its utmost perfection. 

 Except at the articular extremities of the bones, all the 

 original cartilage rapidly disappears ; this matrix is suc- 

 ceeded by marrow, which is in many cases completely 

 absorbed, giving rise to air-cavities, which open into the 

 nearest air-sac. 



The endo-skeletal palatine bones are ossified to a great 

 extent whilst the tissue is still formed of young and soft 



cells, before the solid hyaline cartilage has had time" to 

 grow, and in many passerine birds the exo-skeletal 

 vomers are grafted upon a pair of cartilages, which, how- 

 ever, also belong to the outer skeleton. No true cartilage 

 has yet been found in the outer ear. Rudiments of the 

 hyoid and first branchial arch exist behind the lower 

 jaw. 



fd 



Diagram of the Chondrocranium of a Passerine Bird, au, auditory capsule ; 

 b,hr, basi-branchial ; bJiy, basi-hyal : c.hy, cerato-hyal ; c.br, cerato- 

 branchial; co, columella; c tr, cornua trabeculae : E, eye; e.br^ epi- 

 branchial ; e.pa, ethmo-palatine ; mn, mandible ; n, nasal capsule ; ot.p, 

 otic process ; q, quadrate ; st^ stapes ; i.pa^ trans-palatine ; tr, trabeculae 



The protrusible and retractile face of the bony fishes 

 is constructed in a manner very similar to that seen in the 

 hinged fore-face of a fowl or parrot ; in these birds, as in 

 the fishes, the pre-maxillaries push the maxillaries aside, 

 cover them over, and keep them from the dentary edge, _ 

 thus converting the bones usually so massive, into feeble 

 " moustache bones " {fissa mystacea, Cuvier). 



Mammals. — Such a hypothesis as that Nature bred 

 either all her birds or all her mammals from one stock is 

 at once upset by the facts presented by the structure of 

 the lowest mammals — the duck-billed platypus and the 

 echidna. Between the mammals and the types which 

 foreshadow them, viz., the Selachians and the Batrachians, 

 there is unfortunately a large chasm ; and moreover, the 

 platypus and echidna refuse to lie fairly in the direction 

 indicated at the lop of this chasm, as they confusingly 

 partake of the characters of the reptile and bird ; as well 

 as those which are peculiarly mammalian. 



The skeletal parts of the mammal are modified in a 

 great number of ways answering to the great variety of 

 their modes of life, and especially in regard to prehension 

 and progression. In the higher kinds there are few or no 

 subcutaneous bones {parosioses) in the body, but the head 

 depends upon these for its construction as much as in the, 

 birds : where there are no clavicles there are no parastose 

 behind the head. 



In the lowest forms (platypus and echidna) there ar 

 three clavicular bones ; in the pangolin (one of the 

 Edentata) the body is covered with large imbricated 

 scales ; the armadillos have strong body armour, which 

 is both bony and horny, as in the tortoises and crocodiles 

 and in certain rodents, such as the South America 

 Ca;logenys and Lophioviys imhausii, the bones of th^ 

 head resemble very much the bony scutes of the lowe' 

 types, for they become granular and almost ganoid on* 

 their outer surface. The endoskeleton and the overlying 

 limb-girdles and limbs are developed to a perfection 

 peculiar to the mammal, the culmination of which is seen 

 in our own species. 



Above the lowest forms, neither in the low marsupials 

 nor in the almost equally low Edentata, do we find the 

 middle collar-bone (inter-clavicle). Moreover, in the 

 large Herbivora with a keeled chest, the paired clavicles 

 also disappear ; in man, monkeys, bats, moles, and shrews, 



