38 CLASS REPTILIA. 



silver, as is seen in various lizards and serpents. They also 

 display the azure of lapis lazuli, and the hues of the emerald, 

 the amethyst, and the turquoise. This variegated armature 

 is more especially splendid in spring, when these reptiles 

 throw off the old epidermis of the preceding year. Then 

 they may be seen, in renewed youth, gliding under the new- 

 born vegetation, glittering in the sun-beams, and essaying 

 their recovered vigour and agility. Among the reptiles with 

 thick skin, this renovation takes place but once a year. In 

 the rattle-snakes, this epidermis of the body sliding towards 

 the tail, remains adherent there in the form of small bells— 

 a marvellous provision of nature, who by this means warns 

 other animals of the approach of these redoubtable serpents, 

 whose poison is the most fatal of all. The batracians, instead 

 of parting with a solid epidermis, detach it in shreds. 



The explication of the change of colour in variable rep- 

 tiles seems to belong to the following mechanism. The skin 

 of all the multi-coloured reptiles is not at all of the nature 

 of the feathers of some birds, which assume different tints 

 according to the various reflections of the light, like silk of 

 certain tissues. But in the skin of which we are speaking, 

 the change of colour, whether partial, total, or instantaneous, 

 is dependent on the will of the animal, or some internal emo- 

 tion, without being affected by the circumstances of position, 

 or the aspect of the light. This skin is fine, demi-transpa- 

 rent, and traversed by an infinity of vessels in all" directions, 

 like the dermis of all other animals. But these reptiles, 

 respiring slowly, have a blackish or violet-coloured blood 

 rather than red, because it is so little oxygenated. Now, 

 according as this blackish blood precipitates itself more or 

 less abundantly into the small capillary vessels of the skin, 

 it will produce these shades more or less deep, varied ecchy- 

 moses, or extravasations, with the other natural humours 



