190 J. D. Dana on an Isothermal Oceanic Chart, 



ther is the winter the coldest season in all parts of the globe, 

 especially near the equator. On this account we do not re- 

 strict the lines to a given month, but make them more cor- 

 rectly the limit of the extreme cold for the year at the 

 place.* Between the line of 74 c north and 74° south of the 

 equator, the waters do not fall for any one month below 

 74° F. ; between 68° north and south, they do not fall be- 

 low 68°. 



There are several reasons why isocrymal are preferable to 

 summer or isotheral lines. The cause which limits the 

 distribution of species northward or southward from the equa- 

 tor is the cold of winter, rather than the heat of summer, or 

 even the mean temperature of the year. The mean tempe- 

 rature may be the same when the extremes are very widely 

 different. When these extremes are little remote, the equa- 

 ble character of the seasons, and especially the mildness of 

 the winter temperature, will favour the growth of species 

 that would be altogether cut off by the cold winters where 

 the extremes are more intense. On this account lines of the 

 greatest cold are highly important for a chart illustrating the 

 geographical distributions of species, whether of plants or ani- 

 mals. At the same time, summer lines have their value ; 

 but this is true more particularly for species of the land and 

 fresh-water streams, and for sea-shore plants. When the 

 summer of a continent is excessive in its warmth, as in North 

 America, many species extend far from the tropics that would 

 otherwise be confined within lower latitudes. But in the 

 ocean, the extremest cold in the waters, even in the Polar 

 regions, wherever they are not solid ice (and only in such 

 places are marine species found), is but a few degrees 

 below 32° F. The whole range of temperature for a re- 

 gion is consequently small. The region which has 68° F. 



* The word isocrymal here introduced is from the Greek tiros, equal, and 

 xpvfAos, extreme cold, and applies with sufficient precision to the lines for which 

 it is used. These lines are not isocheimal lines, as these follow the mean winter 

 temperature ; and to use this term in the case before us, would be giving the 

 word a signification which does not belong to it, and making confusion in the 



