EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS 499 
indicates clearly that during the Pliocene, when these gravels were 
being laid down, the climate of Norton Sound, now subarctic, was not 
colder than that of North Japan or the Aleutian Islands where the 
sea remains unfrozen throughout the entire year. ‘This agrees well 
with the evidence from the marine Pliocene of the northeastern corner 
of Iceland, which has afforded over one hundred species, of which 
seventy-four are said to be common to the Crag fauna of the British 
Islands, corresponding to an annual mean air temperature not lower 
than 42° F., while it is hardly necessary to say that the present 
conditions in north Iceland are purely Arctic. A little patch of 
Pliocene at Gay Head, Mass., afforded a fragment of the genus 
Corbicula, now warm temperate in its distribution; while the older 
of the deposits at Sankoty Head, Nantucket, as well as those at Nome, 
show that some of the species which ranged at that period from Bering 
Sea to the North Atlantic are now strictly confined to temperate 
waters in their respective hemispheres. 
I have given most of my time to the relations of temperature to 
faunas, as this is the most important, pervasive, and obvious factor 
of the modifying environment, but there are a few others which may be 
briefly alluded to. 
The question of food is next in importance to temperature. It 
is true that the ocean almost everywhere is a generous provider for its 
inhabitants, so that only special scrutiny reveals important differences 
in the food supply, a large part of which is furnished by almost 
microscopic animals. Yet it has been conclusively shown that in 
places where a persistent movement brings constantly fresh supplies 
of food and well-aerated water, as on the continental slope washed 
by the Gulf Stream, or where the periodical ebb and flow of the tides 
do the same thing on a smaller scale—there the oceanic population 
flourishes with especial vigor and abundance. Near the shores a 
special quota of plant-feeders live, in their turn furnishing provender 
for carnivorous species. The distribution of plant food in the shape 
of algae thus governs the distribution of the phytophagous species. 
We find on the basalts, andesites, and recent lavas of the Aleutian 
chain of islands, enormous groves of kelp and meadows of olivaceous 
rock-weed. Whether because of something in the chemical com- 
position of these rocks, or otherwise, the red and green seaweeds are 
