illustrating the Distribution of Marine Animals. 191 



for its winter temperature, has about 80° for the hottest 

 month of summer ; and the line of 56° F. in the Atlantic, 

 which has the latitudes of the state of New York, follows 

 the same course nearly as the summer line of 70° F. In each 

 of these cases the whole extent of the range is small, being 

 twelve to fourteen degrees.* 



In fresh-water streams, the waters, where not frozen, do 

 not sink lower than the colder oceans, reaching at most but 

 a few degrees below freezing. Yet the extremes are greater 

 than for the ocean ; for in the same latitudes which give for 

 the ocean 56° and 70° F. as the limits, the land- streams of 

 America range in temperature between 30° and 80° F., and 

 the summer warmth, in such a case, may admit of the deve- 

 lopment of species that would otherwise be excluded from 

 the region. 



While, then, both isocrymal and isotheral lines are of im- 

 portance on charts illustrating distribution over the conti- 

 nents, the former are pre-eminently important where the 

 geography of marine species is to be studied. 



The lines of greatest cold are preferable for marine spe- 

 cies to those of summer heat, because of the fact, also, that 

 the summer range of temperature for thirty degrees of lati- 

 tude either side of the equator is exceedingly small, being 

 but three to four degrees in the Atlantic, and six to eight de- 

 grees in the Pacific. The July isothermal for 80° F. passes 

 near the parallel of 30° ; and the extreme heat of the equa- 

 torial part of the Atlantic Ocean is rarely above 84°. The 

 difficulty of dividing this space by convenient isothermals 

 with so small a range is obvious. 



It is also an objection to using the isotheres, that those 

 towards the equator are much more irregular in course than 

 the isocrymes. That of 80° for July, for example, which is 

 given on our map from Maury's chart, has a very flexuous 

 course. Moreover the spaces between the isotheres fail to 

 correspond as well with actual facts in geographical distri- 



* Moreover, the greatest range for all oceans is but 62° of Fahrenheit, the 

 highest being 88°, and the lowest 26° ; while the temperature of the atmosphere 

 of the globe has a range exceeding 150°. 



o 2 



