REPTILES. 



modified Reptiles. At the present day the two groups are, indeed, somewhat 

 widely sundered; and it is only by the study of forms long since extinct that 

 we are enabled to grasp the intimate relationship that exists between them. 

 That Birds are the descendants of Reptiles may accordingly be taken for granted, 

 although we are still unacquainted with the immediate links connecting the two 

 classes. In another direction Reptiles are, however, connected through other extinct 

 forms with the Amphibians ; while from these intermediate, half - Reptile, half- 

 Amphibian creatures, it is probable, as elsewhere mentioned, that Mammals have 

 originated. As we shall point out later on, Amphibians are also intimately con- 

 nected with the class of Fishes, and we thus see how closely allied are all the 

 classes of the Vertebrates, and how difficult is the task of the naturalist to dis- 

 tinguish them satisfactorily one from another when the whole of the extinct forms 

 are taken into consideration. It is, indeed, solely from the still imperfect condition 

 of our knowledge of the past that we are enabled to formulate any definitions at 



all, for had we the whole chain of 

 organised nature before us, it will be 

 obvious that no breaks would exist, 

 but that every group would pass by 

 imperceptible degrees into the earlier 

 one from which it originated. 



Proceeding to the consideration 

 of what constitutes a Reptile, as 

 distinct from any other animal, we 

 may first point out some of the 

 features in which Reptiles agree 

 with Birds, and thereby differ from 

 Mammals. In the first place, the 

 skull articulates with the first A^ertebra 

 by a single knob, or condyle (V of 

 the figure) ; while each half of the 

 lower jaw is composed of several 

 distinct bones; and the whole lower 

 jaw articulates with the skull by the 

 intervention of a separate quadrate-bone. 1 Then, again, both agree in that the 

 appendages developed from the outer layer of the skin never take the form of 

 hairs, while the young are not nourished by means of milk secreted by special 

 glands on the body of the female parent, neither are gills developed at any period 

 of life, throughout which respiration is effected by means of lungs. A further 

 resemblance is shown in the position of the ankle-joint between the upper and lower 

 rows of small bones entering into the composition of that part of the skeleton. In 

 producing their young from eggs (sometimes retained within the body of the parent 

 until hatched), Reptiles resemble not only Birds, but likewise the lowest Mammals ; 

 with which they also agree in the nature of the investments surrounding the 

 embryo. As regards the distinction between the two groups, Reptiles are broadly 



1 In the figure the quadrate-hones are the prominences at the hinder external angles on either side of the 

 letter N. 



LOWER AND UPPER SURFACES OF THE SKULL OF 

 A CROCODILE. 



X, aperture of the internal or posterior nostrils ; 0, 

 sockets of the eyes ; P, vacuities of the palate ; T, frontal 

 vacuities, or fossae ; V, condyle of the occiput. 



