2o8 DESIGN IN NATURE 



the crocodile. There is, therefore, a fundamental identity of plan in the construction of the pelvis of both bird and 

 reptile ; though the differences in form, relative size, and direction of the corresponding bones in the two cases are 

 very great. 



" But the most striking contrast between the two lies in the bones of the leg and of that part of the foot termed 

 the tarsus, which follows upon the leg. It can be shown by the study of development that the bird's pelvis and 

 liind limb are simply extreme modifications of the same fundamental plan as that upon which these parts are 

 modelled in reptiles. 



" On comparing the pelvis and hind Hmb of the Orniihoscelida with that of the crocodile, on the one side, and 

 that of the bird on the other, it is obvious that it represents a middle term between the two. Taking all these 

 facts together, it is obvious that some of these animals may have walked upon their hind legs, as birds do. In 

 fact there can be no reasonable doubt that one of the smaller forms of the Ornithoscelida, Compsognathus, the almost 

 entire skeleton of which has been discovered in the Solenhofen slates, was a bipedal animal. As Compsognathus 

 walked upon its hind legs, it must have made tracks hke those of birds. And as the structure of the limbs of 

 several of the gigantic Ornithoscelida, such as Iguanodon, leads to the conclusion that they also may have constantly, 

 or occasionally, assumed the same attitude, a peculiar interest attaches to the fact that, in the Wealden strata of 

 England, there are to be found gigantic footsteps arranged in order like those of the Brontozoum, and which, there 

 can be no reasonable doubt, were made by some of the Ornithoscelida, the remains of which are found in the 

 same rocks. 



" We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with 

 paw-like fore-limbs and long tails. The evidential value of the facts I have brought forward must be neither over 

 nor under-estimated. It is not historical proof of the occurrence of the evolution of birds from reptiles, for we have 

 no safe ground for assuming that true birds had not made their appearance at the commencement of the Mesozoic 

 epoch. There is another series of extinct reptiles which may be said to be intercalary between reptiles and birds, 

 in so far as they combine some of the characters of both these groups ; and which, as they possessed the power of 

 flight, may seem, at first sight, to be nearer representatives of the forms by which the transition from the reptile to 

 the bird was effected, than the Ornithoscelida. 



" These are the Pterosauria, or Pterodactyls, the remains of which are met with throughout the series of Mesozoic 

 rocks, from the Lias to the Chalk, and some of which attain a great size, their wings having a span of eighteen or 

 twenty feet. These animals, in the form and proportions of the head and neck relatively to the body, and in the 

 fact that the ends of the jaws were often, if not always, more or less extensively ensheathed in horny beaks, remind 

 us of birds. Moreover, their bones contained air cavities, rendering them specifically lighter, as is the case in most 

 birds. The breast-bone was large and keeled, as in most birds and in bats, and the shoulder girdle is strikingly 

 similar to that of ordinary birds. But it seems to me that the special resemblance of pterodactyls to birds ends 

 here, unless I may add the entire absence of teeth which characterises the great pterodactyls (Pteranodon) dis- 

 covered by Professor Marsh. All other known pterodactyls have teeth lodged in sockets. In the vertebral column 

 and the hind-limbs there are no special resemblances to birds, and when we turn to the wings they are found to 

 be constructed on a totally different principle from those of birds. 



" There are four fingers. These four fingers are large, and three of them — those which answer to the thumb and 

 two following fingers — are terminated by claws, while the fourth is enormously prolonged, and converted into a 

 great jointed style. This finger had the office of supporting a web which extended between it and the body. An 

 existing specimen proves that the fingers supported a vast web like that of a bat's wing ; in fact, there can be no 

 doubt that this ancient reptile flew after the fashion of a bat. 



" Thus, though the pterodactyl is a reptile which has become modified in such a manner as to enable it to fly, 

 and therefore, as might be expected, presents some points of resemblance to other animals which fly ; it has, so to 

 speak, gone off the fine which leads directly from reptiles to birds, and has become disquahfied for the changes 

 which lead to the characteristic organisation of the latter class. The occurrence of historical facts is said to be 

 demonstrated, when the evidence that they happened is of such a character as to render the assumption that 

 they did not happen in the highest degree improbable. Those who have attended to the progress of palteontology 

 are aware that evidence of the character which I have defined has been produced in considerable and continually- 

 increasing quantity during the last few years. It is obviously useless to seek for such evidence except in localities 

 in which the physical conditions have been such as to permit of the deposit of an unbroken or but rarely inter- 

 rupted series of strata through a long period of time ; in which the group of animals to be investigated has 

 existed in such abundance as to furnish the requisite supply of remains. It so happens that the case which, at 

 present, most nearly fulfils all these conditions is that of the series of extinct animals which culminates in the 

 horses ; by which term I mean to denote not merely the domestic animals with which we are all so well acquainted 



