ALLEN ON GEOGRAPHICAL DISTRIBUTION OF MAMMALS. 373 



a somewhat similar distribution. While a few genera are restricted 

 respectively to the Northern and Southern waters, the larger unmber 

 are common to both, though represented by different species in the two 

 regions, while they are (in some cases at least) absent from' the inter- 

 vening tropical seas. A large proportion of the Denticete, or Toothed 

 Whales (Dolphins, Porpoises, Eorquals, etc)., are either limited to the 

 warmer seas or have there their chief development, quite a number of 

 genera being peculiar to the tropics. Others, however, like Monodon, 

 are eminently boreal, while others, like Beluga, are common to the colder 

 w'aters both north and south of the tropics. In most cases, however, 

 we know as yet too little respecting the range of the diflerent species 

 and genera of Cetacea to be able to make much use of them in deter- 

 mining questions in geographical zoology. 



This similarity between the marine life of the Arctic and Antarctic 

 Eegions evidently indicates that the forms common to the two had a 

 common origin, and, at some former period, a continuous, probably cir- 

 cumtropical, distribution, and that on the increase of temperature in 

 the intertropical regions, through well-known geological causes, they 

 sought the more compatible cooler waters toward the poles. The 

 similarity of the Arctic and Antarctic marine life is also a feature that 

 sharply differentiates the fauna of the South Circumpolar Eealm from 

 that of the South Temperate and Tropical Zones. 



Ill— GENERAL SUMMARY. 



As stated at the beginning of the present paper, one of the chief topics 

 here proposed for discussion was the influences and laws which govern 

 the distribution of life, — whether it is or is not co-ordinated with climatic 

 zones, and governed in a large degree by climatic conditions, and espe- 

 cially by temperature. In fact, so generally is temperature recognized 

 by the leading writers on the distribution of marine life that it seems 

 superfluous to reiterate or emphasize this principle. That the zones of 

 life should be perhaps a little less obvious over the land-areas, — in con- 

 sequence of the diversity of contour resulting from differences of eleva- 

 tion, and the interruptions and exceptional conditions. due to mountaiis 

 chains and high plateaus, — than over the oceanic expanses, is naturally 

 to be expected. That there is, however, a similar correspondence between 

 climatic belts and the zones of life seems to me abundantly evidenr. 

 As has been already shown, the broader or primary zones are, first, ais 

 Arctic or North Circumpolar Zone, embracing the arctic, subarctic, and 

 colder temperate latitudes of the northern hemisphere, throughout the 

 whole of which area there is a marked homogeneity of mammalian life, 

 as well as of animal and vegetable life in general ; secondly, that below 

 this there is a broad belt of life, which, in its general fades, is distinctive 

 of the temperate and warm-temperate latitudes, and that these two 

 -zones of life are far more closely related inter se than with the life of the 

 intertropical regions, with which regions they may be collectively con- 

 trasted, and together receive the appropriate name of '^ Arctogaia^^ ; 



