2 REPTILES 



species like the Australian frilled lizard, which at times run on 

 their hind legs, form in some degree an exception ; but in the 

 main the diagnosis is true. 



Seeking for something more nearly approaching a scientific 

 definition, and confining our attention to the living members 

 of the class, it may be affirmed that reptiles are cold-blooded 

 vertebrates, unprovided with hair or feathers, which breathe 

 atmospheric air by means of lungs, and do not undergo a marked 

 transformation, or metamorphosis, after leaving the egg, whereby 

 they pass from gill-breathing to lung-breathing creatures. By 

 the first two characters they are distinguished from mammals 

 and birds ; by the third, coupled with the circumstance that 

 their limbs do not partake of the character of fins, they are 

 differentiated from fishes ; while in the fourth feature they differ 

 from amphibians in general, despite the fact that certain more 

 specialised members of the latter have " slipped " some or all of 

 the early stages of development. 



Another important feature of modern reptiles, and one also 

 available in the case of their fossil relatives, is that the skull is 

 articulated to the first vertebra by means of a single, although 

 tripartite, knob, known as the occipital condyle ; this character 

 distinguishing them from mammals and amphibians, although 

 not from birds. 



When we endeavour to go much further in regard to differ- 

 entiating both recent and fossil reptiles from the other three 

 classes of terrestrial vertebrates, we shall be met with increas- 

 ing difficulties. For it is manifest that while reptiles are de- 

 scended from amphibians, it is equally clear that reptiles 

 themselves have given origin on the one hand to birds and on 

 the other to mammals. Consequently, were the whole scheme 

 of animated nature displayed before our eyes, we should expect 

 to find amphibians with a single occipital condyle formed by the 

 development of a median element between the two lateral ones, 

 and without a metamorphosis or, what is the same thing, rep- 

 tiles with a metamorphosis. On the other hand, we should 

 look for reptile-like creatures approximating more or less 

 closely to birds, and acquiring at some stage of their develop- 

 ment wings of a bird-like type, feathers, and warm blood. In- 

 deed, for all we know, pterodactyles, although off the bird-line, 

 may have been (and very probably were) warm-blooded, 



