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 
