92 THE MAIN CURRENTS OF ZOOLOGY 



The Animal Series. In a certain sense animals 

 constitute a series, at the lower end of which are the 

 simple Protozoa and at the upper end the Mammals. 

 However graphic this conception may appear, it 

 needs to be taken with proper qualifications. An- 

 imals do not by any means form a connected series. 

 There is much overlapping of different groups and 

 there are various perplexing offshoots as in the case 

 of the Echinoderms. Animals constitute a series in 

 the sense that they have a common ancestry and that 

 the higher forms are derived from the lower by a 

 process of descent, but adaptations to different con- 

 ditions of life have brought about many diversities. 

 The significant point is that they do not stand as 

 separate creations but are all related structurally, 

 physiologically and psychologically. 



We may say, the most striking general feature of an 

 animal is that it has not arisen independently, but 

 that it exhibits a genetic similarity with other mem- 

 bers of the animal kingdom. The recognition of the 

 common genealogy of animals led to the doctrine of 

 organic evolution. It also prepares us to understand 

 that the study of a few selected types from the differ- 

 ent natural groups, with an analysis of their struc- 

 ture, their life histories and their relations to sur- 



