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234 



STALK-EYED CRUSTACEA. 



of Mexico^ and that this connection was not completely severed till late in 

 the Tertiary period. It is obvious that the former uninterrupted sweep of 

 the equatorial current from the Atlantic into the Pacific must have served as 

 an important agent in disseminating tropical types around the earth. 



Littoral species of the cold and temperate zones have this advantage over 

 tropical types in the matter of distribution on north and south lines: the 

 temperature of the sea rapidly falls in passing from the surface downward^ 

 so that even under the equator a temperate degree of heat is found at a mod- 

 erate depth. Availing themselves of this, many littoral species of the North 

 and South Temperate realms, by moving into the deeper and colder waters 

 have extended their range toward the equator. For example, Cancer horealisy 

 whose normal range as a strictly shore species is limited on the south by the 

 New England coast, was found during one of the cruises of the ^' Blake " in 142 

 fathoms off the coast of South Carolina, living in a temperature of 56J F. 

 under a surface temperature of 81°. Eluding in this way the fatal heat of 

 the tropics, certain species of the temperate zone have actually passed under 

 the equator and invaded the opposite hemisphere. Cancer longipes, a shore 

 crab of Chile, was dredged by the ^' Albatross " in the Gulf of Panama at a 

 depth of 210-286 fathoms, above the seventh parallel of north latitude. The 

 bottom temperature here was 46° to 49° F., while the surface temperature 

 was 72^-74°. The extreme surface temperatures at Valparaiso are given as 

 62° and 62°.* Platymera g aitdichaiidii is another crab long known to nat- 

 uralists as a native of the shores near Valparaiso. This species also w\as found 

 by the '^Albatross" in the Gulf of Panama, living at a depth of 127 fathoms 

 in a temperature of 56° (surface temperature, 74°). The same species was 

 secured during another cruise of the ^'Albatross" much further north, off 

 the California coast from the latitude of San Diego to San Francisco. It is 

 of interest to note that near the northern limit it was fomul even at as slight 

 a depth as 26 fathoms, where it enjoyed the congenial temperature of 58°. 



In this way, doubtless, it has come about that many littoral genera 

 (e. g. Cancer and Lithodes) of the Arctic and North Temperate regions are 

 represented on the shores of corresponding latitudes In the southern hemi- 

 sphere, albeit they are unknown from the vast stretch of intervening coast. 



This extension of the range of Arctic and Temperate littoral animals 

 towjird the equator in the cold oflF-shore waters finds a close parallel in the 

 distribution of land animals. I refer to the well known Influence which alti- 



^ The xncans of the coldest and wannest tlurty coimeeutivc days of the year. 



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