90 CLASSES OF VERTEBEATA : 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 Ver- 

 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 Vertebrated 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 Vertebrata 

 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. 



