12 REPTILES AND BIRDS, 
The respiration of Reptiles and some of the Batrachians, like 
that of Birds and Mammals, is aérial and pulmonary, but it is much 
less active. Batrachians have, in addition, a very considerable 
cutaneous respiration. Some of them, such as Toads, absorb more 
oxygen through the skin than by the lungs. Their circulation is 
imperfect, the structure of the heart only representing one ventricle ; 
the blood, returning after a partial regeneration in the lungs, mingles 
with that which is not yet revivified: this mixed fluid is launched 
out into the economic system of the animal. Thus Reptiles and 
Batrachians are said to be cold-blooded animals, more especially the 
former, in which the respiratory organs, which are a constant source 
of interior heat, are only exercised very feebly. Owing to this low 
temperature of their bodies, Reptiles affect warm climates, where 
the sun exercises its power with an intensity unknown in tem- 
perate regions ; hence it is that they abound in the warm latitudes 
of Asia, Africa, and America, whilst comparatively few are found 
in Europe. ‘This is also the cause of their becoming torpid in our 
latitudes during the winter, not having sufficient heat in themselves 
to produce reaction against the external cold, re-awakening only 
