90 
CLASSES OF VERTEBRATA :—MAMMALS. 
with one exception, they have all red blood (§ 226); and 
that they possess a complex apparatus for circulating this 
through the body. 
76. The four principal modifications under which the Yer- 
tebrated type presents itself, constituting the classes of Mam¬ 
mals, Birds, Reptiles, and Fishes, are respectively character¬ 
ised by the mode in which the principal functions of life are 
performed in each.* Thus there are some Yertebrated animals 
which produce their young alive, and which nourish them 
afterwards by suckling; while the greater part rear them 
from eggs which contain a store of nutritive matter, and 
do not afford them any further nourishment from their own 
bodies. Again, some breathe air; whilst others live con¬ 
stantly in water, and have no direct communication with the 
atmosphere. Some, moreover, have the power of keeping 
up a high temperature, so that their bodies always feel warm 
to the touch ; whilst the temperature of others varies with 
that of the atmosphere, so that their bodies give a feeling of 
coldness : the former are termed warm-blooded —the latter 
cold-blooded. There is a like difference in their mode of life; 
some of them being destined to live on the surface of the 
earth, whilst others are chiefly inhabitants of the air, and 
others again are the tenants of the ocean. 
77. Mammals are distinguished from all other Yertebrata 
by the first of the characters just adverted to; being the only 
animals that produce their young alive, and nourish them 
afterwards by suckling. Like Birds and Reptiles, they 
breathe air by means of lungs ; and, in common with Birds, 
they are warm-blooded and have a complete double circula¬ 
tion of their blood, carried on by a heart with four cavities. 
They are for the most part quadruped (that is, four-footed), and 
are destined to live upon the surface of the earth; but Man, 
and the Apes that approach nearest to him, are biped , having 
the power of walking on two limbs, and of using the others 
for different purposes ; whilst the Bat tribe have the two 
arms converted into wings, which enable them to fly through 
the air like birds (for which the older naturalists mistook 
* Many Zoologists range the Frogs and their allies in a separate class, 
under the name of Amphibia; but when looked at from a physiological 
point of view, the author does not see that they require to be separated 
from the true Reptiles. 
