GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1453 



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 in 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 

 development of species that would otherwise be excluded from the 

 region. 



While then both isocrymal and isothermal lines are of importance on 

 charts illustrating distribution over the continents, 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 species to those 

 of summer heat, also because of the fact that the summer range for 

 30 of latitude either side of the equator is exceedingly small, being 

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

 the Pacific. The July isothermal for 80 F. passes near the paral- 

 lel of 30 ; and the extreme heat of the equatorial 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 

 distribution. The courses of the cold water currents are less evident 

 on such a chart, since the warm waters in summer to a great extent 

 overlie the colder currents. 



It is also to be noted that nothing would be gained by making the 

 mean temperature for the year, instead of the extremes, the basis 

 for laying down these lines, as will be inferred from the remarks 

 already made, arid from an examination of the chart itself. 



The distribution of marine life is a subject of far greater simplicity 



* 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. 



364 



