70 CLASSIFICATION AND CREATION. 
of Reptiles will show that they too are true to 
this structural plan. These are the typical char- 
acters of the whole branch, and exist in all its 
representatives. 
What now are the different modes of express- 
ing this structural plan that lead us to associate 
certain Vertebrates together in distinct classes ? 
Beginning with the lowest class, — the Fishes are 
cold-blooded, they breathe through gills, and they 
are egg-laying; in other words, though they have 
the same general structure as the other Verte- 
brates, they have a special mode of circulation, 
respiration, and reproduction. The Reptiles are 
also cold-blooded, though ‘their system of circula- 
tion is somewhat more complicated than that of 
the Fishes; they breathe through lungs, though 
part of them retain their gills through life; and 
they lay eggs, but larger and fewer ones than the 
Fishes, diminishing in number in proportion to 
their own higher or lower position in their class. 
They also bestow greater care upon their offspring 
than most of the Fishes. The Birds are warm- 
blooded and air-breathing, having a double cir- 
culation ; they are egg-laying, like the two other 
classes, but their eggs are comparatively few in 
number, and the young are hatched by the moth- 
er and fed by the parent birds till they can pro- 
vide for themselves. 
The Mammalia are also warm-blooded and 
