2 
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 vertebra 
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 x 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 iV. 
LOWER AND UPPER SURFACES OF THE SKULL OF 
A CROCODILE. 
• iV, 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. 
