INTRODUCTION 15 



all, or nearly all, smaller seaweeds. The fact, how- 

 ever, is sufficiently striking, that there are 54 species 

 occurring in the two polar areas, which have been 

 separated by a heat-belt so long as there has been 

 climate of any sort on the globe, and if we add the 

 even more striking resemblance of the pelagic forms, 

 the agreement needs some theory to account for it. 

 Marine zoologists have a similar difficulty to face. 

 Blandet, and again recently Dr. John Murray, have 

 brought forward the following interesting theory. In 

 Carboniferous times, they hold, that " the surface tem- 

 perature of the sea could not well have been less 

 than about 70 F., and the same temperature and 

 the same marine fauna prevailed from equator to 

 poles, the temperature not being higher at the equa- 

 tor. ... In early Mesozoic times cooling at the poles 

 and differentiation into zones of climate appear to 

 have commenced, and temperature conditions did not 

 afterwards admit of coral reefs in the polar area, but 

 the colder, and hence denser, water that in conse- 

 quence descended to the great depths of the ocean 

 carried with it a large supply of oxygen, and life in 

 the deep sea became possible for the first time. 

 There have been many speculations as to how a 

 nearly uniform temperature could have been brought 

 about in sea-water over the whole surface of the 

 earth in early geological ages, as well as to how 

 sufficient light could have been present at the poles 

 to permit of the luxuriant vegetation that once 

 flourished in these regions. The explanation that 

 appears to me the most satisfactory is the one which 

 attributes these conditions to the very much greater 



