THE MAKINE FAUNA OF THE FALKLAND ISLANDS. 73 



a universal littoral marine fauna." When cooling set in at the Poles, 

 then the animals with pelagic larvse would be killed out, or be forced to 

 migrate towards the warmer tropics. By limiting their reproductive 

 process to the summer season, some of the organisms with free 

 swimming larvae would live on in the temperate regions. With the 

 disappearance of the shallow-water fauna from the polar regions, its 

 place would be occupied by organisms from the deeper mud line, few of 

 which have pelagic larvse. In this way the similarity and, in some 

 cases, identity between the polar faunas, and the likeness of many 

 shallow-water polar animals to deep-sea species, might be explained. 



The cooling of the w r aters at the Poles would cause vast migrations 

 of forms towards the warmer seas, where metabolic changes would be 

 greater ; this would cause the struggle for existence to be intensely 

 severe in the tropics, and would result in a rapid formation of species, 

 w T hile 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 fauuas. 



Ortmann. 1 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 primitive 

 materia], 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 capability of the inhabitants of the Poles. He does 

 not think that identical species can result in both polar seas from a 



1 Ortmann "Ueber ' Bipolaritat ' in der Verbreitung mariner Thiere." 

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



