Animals Before Man 



further notice other than to say that its mem- 

 bers are almost as uniform in the matter of in- 

 ternal structure as they are in external appear- 

 ance, and that both are modified according as 

 they fly, run, or swim. The divisions of this 

 class are not so sharply defined as are those of 

 other groups of vertebrates, but the crow and 

 ostrich may be taken as representatives of the 

 two principal subdivisions. This is not merely 

 because the one can fly and the other can not — 

 for there are birds related to the ostrich which 

 possess the power of flight, w^hile certain rela- 

 tives of the crow^ are flightless — but on account 

 of peculiarities found in the skull and hip 

 bones of these birds. 



Keptiles, too, are fairly w ell defined in the 

 minds of most of us, although some uncertainty 

 may now and then exist as to whether or not 

 the Amphibia should be included with them. 

 Crocodiles, lizards, snakes, and turtles are fa- 

 miliar and typical examples of this class, but, as 

 we shall see later on, the largest and most 

 striking members of the group, comprising 

 hundreds of species and constituting several 



36 



