EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 407 



occurring in the extra-tropical regions of the northern and southern 

 hemispheres, and wholly unknown from the intervening tropical zone. 

 Again, a list has recently been published giving 54 species of marine 

 Algse common to the northern and southern oceans, and not occurring 

 within the intervening tropical belt. 1 In fact, the arctic and antarctic 

 marine faunas and floras, geographically as wide asunder as the poles, 

 are generally more closely related to each other than to any interven- 

 ing fauna or flora. This is all the more remarkable when we remember 

 that, with the exception of a few pelagic, brackish water and deep sea 

 species, there is hardly a species of marine Metazoa common to the 

 east and west coasts of Africa within the tropics. 



In order to give a rational explanation of these remarkable facts in 

 the distribution of marine organisms at the present time, as well as of the 

 presence of tropical fossils in Paleozoic and even later geological strata 

 within the polar areas, it seems necessary to assume that at one time 

 there was a very different distribution of heat and light over the sur- 

 face of the globe than what obtains at the present time. A uniform 

 high temperature all over the surface of the globe in the early stages 

 of the earth's history is required to explain these phenomena. In later 

 Mesozoic times a gradual cooling at the poles appears to have set in, 

 and slowly brought about the destruction of a large number of the 

 shore and- shallow- water animals, especially those which secreted large 

 quantities of carbonate of lime or were provided with pelagic or free- 

 swimming larvse. This weeding out of numerous species in the polar 

 areas, from a fauna which must have much resembled the coral-reef 

 fauna of the present time, accounts for the relatively small number of 

 species which we now find in polar waters, and, through lessened com- 

 petition, for the relatively large number of individuals belonging to 

 some of these species. In still later times, when polar lands became 

 covered with ice and snow and when glaciers descended at almost all 

 points into the ocean, shallow-water organisms appear to have taken 

 refuge in the deep sea, and a migration of polar animals toward the 

 equator was initiated over the floor of the ocean. This may account 

 for the relatively more abundant fauna in the great depths of the 

 Southern Ocean, as indicated by the Challenger's investigations. The 

 large numbers of pelagic animals which are continually being killed 

 through the mixture of surface currents of different origins between 

 latitudes 40° and 50° south, falling to the bottom, provide an abundant 

 supply of food for deep-sea animals, and the large quantity of oxygen 

 taken down by descending currents from the cold surface waters pro- 

 duces further favorable conditions of life in these great depths. 



In discussing the causes of the distribution of organisms over the sur- 

 face of the earth at the present time, or the geographical distribution of 

 fossils in Paleozoic rocks, it is too often assumed that the relations 



1 Murray and Barton. Phycological Memoirs from the British Museum, Lonclou, 

 1895. 



