INTRODUCTION. 



A mammal represents the highest development in the animal 

 kingdom and may be broadly defined as a warm-blooded vertebrate 

 animal more or less covered with hair,* which suckles its young. 



To describe it more fully, it is an air-breathing, warm-blooded 

 vertebrate, differing from all other animals except birds in having a 

 four-chambered heart and a complete double circulation, but unlike 

 birds the red blood corpuscles are non-nucleate. The heart and lungs 

 are separated from the abdominal cavity by a muscular diaphragm. 

 With rare exceptions the jaws are armed with teeth. The skull articu- 

 lates with the first vertebra (atlas) of the vertebral column at two sep- 

 arate points (occipital condyles). Each half of the lower jaw consists 

 of a single bone which articulates directly with the skull. The female 

 is furnished with teats,t and the young are nourished at first by milk 

 secreted in the milk glands (mammce) of the mother, whence the name 

 of the class. The young are born in various stages of development. 

 Some, like those of the Opossum, are rudimentary at birth; while others 

 are developed but hairless, blind and helpless; and still others when 

 born are fully clothed with hair, with eyes open, and are able to stand 

 and move about from the first. 



Mammals differ strikingly in appearance, habits and size: some are 

 aquatic, fish-like and practically hairless, such as the Whales, Por- 

 poises, etc. ; while others, like the Bats, are provided with wings enabling 

 them to fiy about in the air. The majority are terrestrial, but some are 

 semi-aquatic; others arboreal, passing much of their lives in trees, and 

 some live in burrows in the ground. Some are diurnal and others 

 nocturnal, while a considerable number cannot be strictly included 

 in either category. Most of our species are active in winter, | but a 



* In marine mammals, such as Whales, while hair is absent in the adult or 

 confined, to a few bristles about the mouth, it is noticeably present in the young. 



t The Monotremes of the Australian region furnish the only exception; the 

 females have mammary glands but no developed teats. 



X The seeming scarcity or absence of many species in winter is due to their 

 mode of life, as comparatively few of them migrate in the strict sense of the word. 

 Some of them, having stored up food in their homes during the summer, remain 

 indoors during the cold weather and come out but little; while others hibernate 

 in winter. Among the latter are the Ground Squirrels (Citellus), Woodchuck, 

 Chipmunks Jumping Mouse, Badger, Raccoon, Bear and Bats. The phenomenon 

 of hibernation is a strange physiological condition peculiar to certain animals, which 

 enables them to thrive in regions where they would otherwise probably starve 

 in winter, were it not for their ability to remain dormant during such periods of 



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