THE FROG AND VERTEBRATES IN GENERAL 159 



Class VI. Aves. Since the Aves, or Birds, are the only animals 

 which possess feathers, this one character serves to differentiate 

 them from all other groups. The main portion of a feather devel- 

 ops in the dermis of the skin, and is covered externally by an epi- 

 dermal layer. They are closely related to the reptilian scale. 

 Birds possess two pairs of limbs, but the fore limbs are highly 

 modified for flying. Even the most primitive fossil birds show this 

 amazing fore limb development, and it persists throughout all 

 species. (W. fs. 84-86, 233.) 



Another interesting and important feature found in this class 

 is the maintenance of a uniform body temperature (homothermal 

 or, commonly, warm-blooded), a condition which is found in only 

 one other group of animals, namely, the Mammals, in which 

 Man is included. In all other classes of Vertebrates and Inverte- 

 brates, the body temperature varies with the environment of the 

 animal (poikilothermal, or, commonly, cold-blooded). Attention 

 should also be called to the fact that all Birds and Mammals possess 

 a four-chambered heart. This condition is found in certain Rep- 

 tiles, but it is not universal in that class. Although teeth can be 

 demonstrated in certain fossil birds, they are lacking in present-day 

 species. 



The Birds represent an extremely homogeneous group, so much 

 so that it is very difficult to find a sufficient number of differen- 

 tiating structural characters to construct a satisfactory scheme of 

 classification for the nearly 20,000 species which are known. Ex- 

 cluding fossil forms, seventeen orders are usually recognized, and 

 the basis of classification depends, for example, upon such rela- 

 tively minor structural features as the character of the feet and 

 beaks. (W. f. 85.) 



Class VII. Mammalia. This large, interesting, and most 

 important class of Vertebrates is characterized by the develop- 

 ment of hair in the skin. Abundant in many species, where it 

 forms a heavy external covering, in certain other types it may be 

 considerably restricted or in an extreme case, such as in certain 

 Whales, be entirely lacking. Another mammalian characteristic 

 is to be noted in the mammary glands, present on the ventral 

 surface of the female, which form a secretion (milk) for nourishing 

 the young after birth. A constant body temperature is main- 

 tained by all Mammals. 



Probably the greatest amount of external variation is to be 



