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,f and the young are nourished at first by milk 
secreted in the milk glands (mamme) 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 fly 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,t 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. 
+ The Monotremes of the Australian region furnish the only exception; the 
females have mammary glands but no developed teats. 
t 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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