442 THE GENERAL PRINCIPLES OF ZOOLOGY 



This very brief survey of the field of palaeontology brings to 

 light a few fundamental biological facts which we shall see later 

 have a great theoretical interest. There is no evidence in facts 

 that all classes of animals came into existence in the same 

 geological period ; on the contrary, the classes which we recog- 

 nize to-day as the most highly organized in the animal series 

 seem to have been the last to make their appearance on the 

 earth. We note, further, that when any group of animals first 

 appears it presents, as a rule, a much more generalized type of 

 structure than its later representatives ; fossil remains of many 

 adult animals possess in many respects the structure of the 

 embryos of existing groups. Thus the older fossil fishes and 

 reptiles had the vertebral column cartilaginous or incompletely 

 ossified. The older remains of reptiles and mammals show that 

 the structure and shape of the skull were like those of the embryo 

 of recent genera. We have seen that many groups have become 

 extinct, just why it is not always easy to say. During the last 

 few hundred years several animals have become extinct through 

 man, and many more are likely to become so in the near future. 

 In the past it has probably been much the same. The stronger, 

 the more voracious, the more cunning animals have preyed on 

 their weaker or more stupid neighbors, and the result may have 

 been extinction. Others may have been unable to meet the 

 changed conditions of climate or temperature to which they were 

 subjected, an inability resulting, perhaps, from too high a degree 

 of specialization along some one line ; and thus extinction has 

 resulted. But whatever the cause, it has probably been, in the 

 majority of cases at least, a gradual one ; the old belief that 

 violent interruptions of the action of natural forces led to the 

 wiping out of entire genera or families has been abandoned. 

 We have already noted some of the reasons why the palaeonto- 

 logical record is so incomplete ; with these factors in mind 

 we marvel not at its incompleteness, but rather at its com- 

 pleteness. 



The attempt has often been made to represent diagrammatic- 

 ally the relations of the various geological periods to one another, 

 and the origin and development of the different groups of ani- 

 mals. Such diagrams are necessarily more or less inaccurate 

 and misleading, but they serve as an aid to the elementary 



