of the Arctic and the Antarctic Faunas. 305 
different requirements in regard to warmth, and were there- 
fore able to live also in temperate latitudes; secondly, 
because, if our latitudes enjoyed a climate of tropical warmth 
in ‘Tertiary times, the torrid zone must have had a hyper- 
tropical climate, which would have annihilated all life 
within it. 
These objections cannot here be considered in detail, but 
the most important refutations of them may be brought 
forward. 
(1) Modern biology has long since admitted that the chief 
factor determining the distribution of plants and of cold- 
blooded animals, and especially marine animals, is to be 
sought for in the conditions of temperature. Other conditions 
of life are, of course, of great influence, but they only ac- 
centuate the state of affairs primarily brought about by the 
temperature. Thus equality of temperature is, ceteris 
paribus, a distribution-bridge, inequality a distribution - 
barrier. The fauna of our tropical surface-water cannot 
spread from the tropics into temperate latitudes, and it is 
contrary to our most firmly established beliefs to assume 
that a fauna with a habit similar to that of the tropical fauna 
of to-day, and with, in the main, the same genera, can have 
lived in a temperate climate in Karly Tertiary times. This 
may be especially illustrated by reference to the reef-corals, 
which form such a characteristic feature of the tropical fauna 
of to-day, precisely because of their invariable sensitiveness 
to less than tropical heat. 
Great probability is lent to this view by the more and 
more pronounced separating-out of the Karly Tertiary fauna 
into zonally disposed faunas, which took place during the 
Tertiary period. This point will be more fully discussed 
later on. 
This view of the climate of the Tertiary period has been 
strongly corroborated by the researches of Murray and Irvine, 
according to which an abundant secretion of lime is only 
possible in a warm climate. ‘That lime in solution is precipi- 
tated only to a slight extent in cold water, but in great 
abundance in warm ‘water, is in itself only a ‘chemical fact ; 
but the circumstance that the animals of the higher latitudes 
secrete little lime, while tropical animals secrete it in abun- 
dance, at once gives the chemical fact a physiological signi- 
ficance. And, according to it, the formation of coral-reefs is 
possible only in water of tropical warmth. 
We believe, therefore, that no change in the amount of, 
warmth required by marine animals has taken place. The 
lovers of warm water, which were unable to endure the 
