^^ ZOOGENESIS ^^ 



other both in the reduction of some of the bodily 

 structures and in the very great development of others. 

 Thus they differ from each other both by subtraction 

 and by addition. To assume that any of the major 

 groups are derived from any of the others is therefore 

 to deny the general application of a well estab- 

 lished principle. 



Another factor which probably has a bearing on 

 the question of the simultaneous development of the 

 major animal groups is the necessity for the main- 

 tenance of a balance between the different forms of 

 life. The necessary check on the excessive increase 

 in the numbers of any type of animal is provided by 

 predacious animals, by parasites, and by various 

 types of animal feeding plants, principally bacteria 

 and fungi. It is impossible to believe that such a 

 check was not as essential at the time of the first 

 appearance of life as it is today. 



The process leading to and resulting in the first 

 appearance of some representative or representatives 

 of each of the major groups may be known as eogenesis. 



Evolution. — We come now to the second question, 

 what has been the history of animal life from its first 

 appearance to the present day? 



The various types of animals included in each single 

 one of the larger major groups — for instance in the 

 vertebrates or backboned animals, the mollusks or 

 the arthropods — have varied very greatly in succes- 

 sive geologic ages. This is shown conclusively by a 

 study of the fossils. If the broader aspects of the 

 living world have from the first remained unchanged, 



