VERTEBRATES — CLARK 297 



covering of feathers (modified scales) and scales (on the legs) ; the 

 skin is dry ; body temperature controlled and constant, high — higher 

 than in the mammals ; breathing by internal lungs, and with extensive 

 internal air sacks ; highly dependent on adequate illumination, espe- 

 cially in the visual band of the spectrum ; mostly diurnal, a few crepus- 

 cular, none truly nocturnal ; oviparous, with internal fertilization, the 

 eggs always with a hard and rigid shell; the young are hatched in 

 various stages of development. 



Mammals. — Terrestrial animal or vegetable feeders, or omnivorous ; 

 with a body covering of hair ; with sweat glands in the skin ; body tem- 

 perature controlled and constant, nearly always higher than the sur- 

 roundings, at least at night, but in the monotremes variable and in 

 many types reduced during hibernation ; breathing by internal lungs ; 

 independent of bright illumination ; diurnal or nocturnal ; viviparous, 

 except for the oviparous monotremes, with internal fertilization, the 

 young born in various stages of development and fed for a varying 

 length of time by a secretion from the body of the mother. 



The basic ecological features of the vertebrate classes may be 

 graphically expressed in the form of a key, as follows : 



KEY TO THE BASIC ECOLOGICAL FEATURES OF THE VERTEBRATE CLASSES 



a 1 . Dependent on a constant and abundant supply of external water ; 

 aquatic or amphibious ; independent of bright illumination ; 

 coldblooded (Ichthyopsida). 

 ft 1 . Strictly aquatic in fresh or saline waters of relatively large 



size Fishes 



6*. Aquatic in strictly fresh waters of small dimensions or tem- 

 porary, or if of large dimensions more or less choked with 

 vegetation, at least when young; as adults mostly emer- 

 gent in regions of high humidity Amphibians 



a 2 . Independent of a constant and abundant supply of external 

 water ; terrestrial. 

 6 1 . Highly dependent on sunlight (Sauropsida). 



c\ Cold-blooded but active and requiring a relatively large 



amount of heat provided by sunlight ; terrestrial Reptiles 



<?. Warm-blooded, independent of heat from the sun ; terres- 

 trial, with the power of flight Birds 



6 3 . Largely independent of sunlight ; warm-blooded Mammals 



In order to illustrate the preceding key it is desirable to designate 

 definite types that may be tentatively regarded as generalized repre- 

 sentatives of the several classes. These types will serve to indicate 

 in each class an approximation to the basic or fundamental complex 

 as contrasted with the fringe or peripheral forms. Just what the best 

 representative of each class would be is, of course, a matter of personal 

 opinion. The following are suggested. 



