INTRODUCTION. 639 
attention has not hitherto been properly directed, is the various modes 
of protection furnished them by nature. We find the Turtles, for the 
most part, provided with a union of the cuticle and skeleton into which 
they can retreat, and which can be closed about them. To see the value 
of this mode of protection, it is but necessary to endeavor to draw out the 
head of one, which will be found to be a very difficult task. In asimilar 
manner, the scales, plates, and spines, 50 common as a dermal covering, 
havea profound significance, and their color is a matter of too great a value 
to be overlooked. The Rattlesnakes and Copperheads are provided with 
fangs, for injecting venom, and the former with a caudal rattle, by the 
vibration of which it may aid in frightening away its foes. But the color 
has an important bearing. The young of the venomous Crotalophorus 
tergeminus, and of the harmless Tropidonoius sipedon and Ophibolus trian gulus, 
bear such a resemblance to each other that it might be considered a case 
of mimicry. However, the colorof these animals is more or less adapted 
to their surroundings. The Green Snake resembles closely the grasg 
through which it crawls, and the same is true also of the Storerias and 
Striped Snakes. Unless closely examined they could easily escape the 
eye of the observer. In like manner the markings of frogs furnishes an 
excellent illustration of adaptation. The green of the Bull-frog and the 
spots on the Leopard Frog need but be mentioned to be appreciated. 
Rana temporaria var. sylvatica, when pursued, conceals itself in grass or 
leaves, which it resembles so clesely as to be discovered with difficulty. 
The Common American Toad (Bufo americanus), might not at first seem to 
have its colors most suitable for protection, and yet when we see it in its 
secluded retreats, under stones and in odd corners, and consider its noc- 
turnal habits, the conviction forces itself upon me, that probably no 
better shades could have been chosen. As desert animals are sand colored, 
and arctic are white, like the snow, so these animals show a decided 
adaptation to their surroundings. 
The best illustration is furnished by the common Tree Toad (Hyla ver- 
sicolor). Like the Chameleon, this little animal can change its color, and 
thus transform itself into a being resembling the limb or branch on 
which it sits. As Milne Edwards* found in the Chameleon, so in this 
there are two sets of cutaneous glands, the one superficial and the other 
deep. The former of these gives it a green coloration, with a golden re- 
flection ; the latter are much thicker and dark. They contain many 
brush-shaped cavities, and the expansion of these towards the circum- 
ference determines the color and causes the green tint to disappear, not 
*Ann, des Sci. Nat., 1834, p. 46. 
