394 THE AMERICAN NATURALIST [Vol. LIV 



occasionally, in both. They may proceed from warmer 

 to colder zones, they may proceed from colder to warmer 

 zones, or they may proceed from a zone of intermediate 

 temperatures to both colder and warmer zones. Where 

 warmer spots or areas exist in the midst of cooler waters, 

 species of warmer zones may exist spotwise in the cooler 

 zone. Where certain portions of the waters of a warmer 

 zone are depressed in temperature by cold currents or 

 upwellings, or, for certain seasons of the year, suffer a 

 general lowering of the temperature, there and then may 

 species from cooler zones be expected to put in an ap- 

 pearance. The extent of such invasions will depend 

 naturally upon the intensity and duration of the unusual 

 temperature. A consideration of the examples I men- 

 tioned as typically eurythermal may serve to make this 

 idea more clear. 



Zostera marina seems, on careful study of its occur- 

 rence and habits on all the coasts where it is found, to be 

 normal to the North Temperate Zone with the mean 

 maxima for the hottest month from 15° to 20° C. If this 

 is the case, we are dealing with a species which extends 

 in both directions from its normal zone. The more 

 northern extensions may be explained by the fact that 

 the very shallow and protected lagoons and interiors of 

 prolonged and narrow bays preferred by this species 

 may have the temperature of their waters raised through 

 the action of the air and of the sun. In such waters, 

 insolation undoubtedly is the most effective agent in 

 raising the temperature as much as 10°-12° C. or even 

 higher. To the south, the invasions of Zostera marina 

 may be assumed to be made possible by the seasonal 

 lowering of the temperature of the waters through the 

 lower winter temperatures, e.g., the winter temperatures 

 on the coast of North Carolina is somewhat under 20° C. 

 and the winter temperature on the coast of West Flor- 

 ida is also somewhat under 20° C. It would be expected, 

 if the seasonal lowering of the waters south of the lower 

 limits of the North Temperate Zone allows the eel-grass 

 to find its normal temperature for fruiting in an earlier 



