INTRODUCTION 



HAT is a mammal? 



The beginner in any study has certain basic facts to acquire, and this is 

 never more true than in an exactly related science such as natural history. 

 It is absolutely necessary to begin here by mastering a few of the broad, under- 

 lying principles, in order to follow the subject diligently. These are not difficult 

 to follow, if the reader takes them up one at a time in regular order; and the 

 first question that would naturally arise in the present volume is, What is a 

 mammal? To answer it, let us turn back for a moment to the beginning of 

 animal life. All living animals fall under one of two heads, or kingdoms, as 

 the naturalist calls them, the Invertehrata and the Vertebrata. The Invertebrata 

 are the lowest forms of life, and are so called because of the fact that they lack a backbone, 

 or spinal column. To this kingdom belong the hosts of creeping, crawling animals such as 

 Worms, Insects, Molluscs, Sponges and Jellyfish. 



The Vertebrate kingdom contains the higher types of life, beginning with such lowly 

 forms as the Lancelets and Tunicates, and running upward in the scale to Man himself. 

 The Vertebrates are characterized by the possession of a backbone, which is cartilaginous 

 in the lower animal types, but formed of true bone in the higher. This kingdom may be 

 divided into five divisions or classes, viz., the Fishes, the Amphibians, the Reptiles, the 

 Birds, and the Mammals. 



HOW MAMM,\LS DEVELOPED 



Mammals are thought by competent authorities to have been developed from reptilian 

 ancestors. They hold this belief because fossil reptiles of a highly specialized type have 

 been found possessing mammal-like characters. The theory, in brief, is that from some 

 verj' active, highly specialized reptile of this type, the first mammal was evolved at an early 

 period. Mammals, however, are not an ancient class, when compared with other Vertebrates, 

 for it is almost certain that the Mammals and the Birds are the latest arrivals of all. 



Mammals differ from other Vertebrates mainly in the following characters : the posses- 

 sion of hair, mammary glands, a high blood temperature, a four-chambered heart, a dia- 

 phragm, a highly developed brain and nervous system, and also some very important 

 characters in the skeleton. Probably the best-known character is the possession of hair to 

 a greater or less degree, varying from coarse bristles to finest fur. This form of outer covering 

 is opposed to that of other classes. For example, the Birds have feathers, the Reptiles 

 scales, the Amphibians are generally naked, and the Fishes have scales. 



A very important character of the Mammals, the one that gives the class its name, is 

 the presence of mammae, or breasts, on the female. These structures are necessary because 

 Mammals bear their young alive and helpless, and they must be fed by the mother until 

 strong enough to take care of themselves. In the other classes the young more often come 

 from an egg, or, if born alive, are more or less able to shift for themselves. 



The high blood temperature and the four-chambered heart are most valuable to the 

 Mammals. Because of this fact alone they would be able to dominate the other classes. 

 Long periods of sustained activity are possible only through a warm and consequently active 

 blood stream. In all the other classes, except the Birds, the members have what is com- 

 monly called cold blood, or, to speak more exactly, a variable blood stream. This results 

 in long dormant or sluggish periods. The warm-blooded Birds, on the other hand, lack the 



