Manchester Memoirs, Vol xlii. (1898), No. 13. 9 



struggle for existence to be intensely severe in the tropics, 

 and would result in a rapid formation of species, while 

 many would become extinct. On the other hand, the 

 metabolism being less in the temperate and colder waters, 

 and the struggle for existence being less severe here than 

 in the warmer waters, there would be less tendency for the 

 species to become modified, and many would remain true; 

 hence the similarity between the North and South tem- 

 perate faunas. 



Ortmann,* whilst acknowledging the possibility of the 

 existence at one time of a universal fauna, contends that the 

 cooling at the Poles did not arrest the capability of 

 variation, but that the bipolar forms now existing must 

 have passed through a greater range of variation than the 

 tropical forms ; in other words, that the tropical fauna has 

 remained more or less true, while the temperate and 

 bipolar forms are derivatives of ancestral forms. 



He admits that a form with a well -developed 

 adaptative faculty may have passed through all the 

 varying conditions of temperature, etc., without becoming 

 extinct. The changes due to climatic conditions being 

 similar at both Poles, two faunas resulted from the primi- 

 tive material, one Arctic, the other Antarctic. 



He also holds that the likeness between the north 

 and south polar faunas, is in many cases a secondary 

 reappearance, and is dependent on the adaptative capa- 

 bility of the inhabitants of the Poles. He does not think 

 that identical species can result in both polar seas from a 

 common descent. He maintains that an exchange of both 

 polar forms can take place tJiroiigJi the deep sea, on the 

 ground that, among Crustacea, the cosmopolitan genus 

 Po7itopJiiliis shows a decided tendency to retire into deep 



* Ortmann " Ueber ' Bipolarital' in der Verbreitung mariner Thiere." 

 Zool.Jahrb. {Abth.f. Syst.) Bd. 9, (1896), p. 571. 



