2i8 W. H. BALL 



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 

 almost wholly absent from them. However, where the granitic 

 masses which form the core of some of the islands (and in other 

 places stand alone, domelike in the sea) are within reach of the waves, 

 we find a special flora of the more bright-colored algae and a special 

 fauna dependent upon them. No matter how isolated the patch of 

 granite, the characteristic animals recur, and in many cases reproduce 

 in their own tints the rosy hue of the plants upon which they depend 

 for food. 



In the abysses where the absence of sunlight excludes plant life 



