ANIMAL LIFE 193 



may be small differences to be detected only 

 by carefully examining fresh samples. He 

 says further that it is very interesting to see 

 from the abundant material now available 

 how some species, both of peridineans and 

 diatoms, are represented in the Arctic and 

 in the Antarctic by very similar but not quite 

 identical forms. 



From the foregoing observations it appears 

 that: 



(1) Species from the far north and far south, 

 which some naturalists consider identical, are 

 regarded by others as quite distinct, even 

 although the differences are slight. 



(2) Some authors look upon bipolarity as 

 limited to identical species, while others apply 

 the term to genera, families and orders. 



(3) Some naturalists consider that there is a 

 direct interchange of the so-called bipolar 

 species by way of the cold deep waters of the 

 tropics, or along the western coasts of con- 

 tinents where the temperature is lowered by 

 cold currents and by upwelling. 



(4) Some authors hold that, as there is a 

 bipolarity in the chemical and physical 

 conditions of the Arctic and Antarctic seas, 

 these similar conditions give rise to what are 

 called vicarious or parallel forms through 

 convergence. 



