Prologue: Why Study Vertebrates ? 3 



is difficult to frame a satisfactory definition of "alive" but it is easy in 

 practice to distinguish living things from those which are not alive. At 

 least, it is easy until we come to the ultramicroscopic realm of such 

 things as filtrable viruses and "giant molecules" which cannot be 

 brought within direct range of the several senses which serve for our 

 intuitive recognition of life in a dog, robin, codfish, earthworm, ameba, 

 or fellow man. 



This thing of being "alive" is accompanied by peculiarities of struc- 

 ture and behavior which are essentially similar in all animals. Our pres- 

 ent considerable knowledge of these vital mechanisms makes it so evi- 

 dent that man and all animals are things of the same general nature 

 as to require that the old phrase "man and animals" be amended to 

 read "man and other animals." Whether man, in the conscious and 

 mental aspect of his life, possesses something which other, or some 

 other, animals lack, or whether this part of his life is merely a more 

 elaborate development of potentialities inherent in perhaps all living 

 things, is an important question. Whatever the answer, man is at least 

 an animal. 



Man does not stand apart from other animals. He is associated with 

 them in many important ways. The association may be helpful, essen- 

 tial, harmful, or disastrous to him. In some of these associations, there- 

 fore, he is an unwilling member. In our relations to insect pests, in- 

 ternal and external parasites, and beasts of prey, either the other 

 animal is the aggressor or else the association is forced upon us by some 

 peculiar set of biologic circumstances. But man takes the initiative in 

 associating with himself certain animals such as the dog, cat, horse, 

 cattle, sheep, and some birds. Some of these "domesticated" animals 

 provide him with important food. Others serve him for transportation 

 or in connection with hunting. But his association with dogs, cats, and 

 horses may contain something even approaching the friendship which 

 may exist between man and man. We feel a certain bond of sympathy 

 with these animals. 



Some animals resemble man more than do others. The central or 

 axial part of the human skeleton is a series of bones (vertebrae) mova- 

 bly jointed together and extending lengthwise of the body (Fig. 3). 

 The series is commonly called the "backbone," otherwise the vertebral 

 column. The dog, cat, and horse have backbones similar to man's — 

 they are vertebrates. If land crabs grew to be as large as dogs, and 

 long-haired caterpillars as large as Persian cats, it can hardly be im- 

 agined that such backboneless animals would become our household 

 pets and companions. We usually lack a cordial fellow-feeling for in- 

 vertebrates. This is not because of their lack of backbone but because 

 they are so radically unlike ourselves in other respects, even to the 



