THE ROYAL NATURAL HISTORY. 
MAMMALS. 
CHAPTER I. 
General Characteristics,— Class Mammalia. 
In describing any group of objects, whether they be artificial or whether they be 
natural, some method of classifying is absolutely essential to a right understanding 
of their relations to one another; and nowhere is this more important than in 
Natural History. To a certain extent such a classification is already made in our 
ordinary language, since we are accustomed to divide the higher animals into several 
distinct primary groups, under the names of Mammals or Quadrupeds, Birds, 
Reptiles, and Fishes; and these primary groups coincide in the main with those 
employed by zoologists. Such a popular classification depends almost entirely 
upon similarity or dissimilarity of outward appearance and form; and although 
this is a good and dependable guide in many cases, it is by no means always 
trustworthy, and may, indeed, frequently lead us into serious error. For instance, 
whales and dolphins are generally associated in the uninstructed mind with fishes, 
whereas, as a study of their internal structure at once reveals, they are really 
Mammals, which have been specially adapted for a purely aquatic life. 
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