2 Pratt, Bipolar Theory. 



of forms obtained by the Challenger expedition, and, by- 

 drawing up a long list of bipolar forms, supported Pfcffer's 

 theory. Furthermore, by a consideration of former different 

 climatic conditions affecting the nature of the surface of 

 the earth, he shows how this relationship may have been 

 brought about. He maintains that if there were once a 

 nearly universal climate over the whole of the ocean, then 

 it is possible that there was a universal littoral marine 

 fauna. The cooling of the earth at the poles would cause 

 vast migrations of forms towards the tropics, where the 

 struggle for existence would be extremely severe, and 

 metabolism would be great. This would result in modi- 

 fication of old, and rapid formation of new, species in the 

 warmer waters. Many forms with free-swimming pelagic 

 larvae, by limiting their reproductive process to the summer 

 season, would be able to live on in the temperate regions, 

 where metabolism would be less than in the warmer waters, 

 and would remain more or less true. Thus the likeness 

 of many littoral, temperate, extra-tropical forms to each 

 other would be explained. 



With the migration of forms from the poles, their 

 place would be taken by organisms from the deeper 

 mud-line, few of which have pelagic larvae. This would 

 explain the likeness between Arctic and Antarctic forms. 



The theory put forward by Pfeffer and strongly 

 supported by Murray, met with considerable opposition 

 from Ortmann, Professor D'Arcy Thompson, and others. 

 In the following pages I have attempted to discuss briefly 

 the evidence for and against the theory. 



Ortmann ('96-'99) contends that the cooling of the 

 waters at the poles did not arrest metabolism, and main- 

 tains that the tropical fauna has remained more or less 

 true, while the temperate and polar forms are derivatives 

 from ancestral forms. He maintains that the likeness 



