2 VERRILL 



Among the fishes the Salmonidae may be mentioned. Among the 

 mollusca some groups, like the limpets, chitons, Buccinidae, and 

 other families, are much more abundant than in any other regions. 



Nemerteans and tubicolous Annelids are also remarkably large and 

 numerous. 



There is every reason for thinking that such groups, including 

 various genera and families of starfishes, have lived and flourished 

 on that coast for many geologic periods, and have developed in that 

 region, by continuous evolution, most of the species found there at 

 present, as well as great numbers that have disappeared. 



Indeed, I am led to conclude that this region has been, for long 

 ages, a center of evolution, from which various generic and specific 

 types of starfishes have been dispersed to other faunal areas, with 

 or without subsequent changes. 



It is not at all improbable that some arctic and boreal species of 

 the North Atlantic originated on the Alaskan coast and have since 

 crossed to the Atlantic by way of the Arctic Ocean, when the arctic 

 climate was warmer. 



Others may have migrated southward to the subtropical and 

 tropical regions of Mexico and Central America, and even to the 

 Antarctic Seas, for some of the deep-water Patagonian and antarctic 

 species show peculiar affinities with the Alaskan species. 



Such migrations, across the tropics, would easily be possible for 

 species having a considerable range in depth, for their congenial 

 temperatures could easily be found at no great depth, even off 

 Panama. 



One reason for the great variety and persistence of the starfishes 

 and other groups on this coast is, no doubt, the fact that the tem- 

 peratures of the waters were not seriously reduced during the 

 Glacial Period, while those of the North Atlantic were rendered so 

 cold that the preexisting fauna was largely exterminated, while the 

 survivors were driven southward, except in the case of eminently 

 arctic and deep-sea species. 



Another very favorable condition is the comparatively equable 

 temperature of those waters, due to the reflex of the great Japanese 

 tropical current of water. Its influence in modifying the climate 

 of the whole coast, south of the Aleutian Islands, at least, is well 

 known. 



For the prosperity of most marine animals, equability of temper- 

 ature is exceedingly favorable, whether high or low, especially at the 

 breeding season. 



