1452 CRUSTACEA. 



The line for 68 F., for example, passes through the ocean where 68 

 F., is the mean temperature for extreme cold weather. January is 

 not always the coldest winter month in this climate, neither is the 

 winter the coldest season in all parts of the globe, especially near the 

 equator. On this account, we do not restrict the lines to a given 

 month, but make them more correctly the limit of the extreme cold 

 for the year at the place.* Between the line of 74 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 below 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 equator is the cold of winter, rather 

 than the heat of summer, or even the mean temperature of the year. 

 The mean temperature may be the same when the extremes are 

 very widely different. When these extremes are little remote, the 

 equable 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 impor- 

 tant for a chart illustrating the geographical distributions of species, 

 whether of plants or animals. 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 sea-shore plants. When the sum- 

 mer 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 Fahrenheit. The whole range of temperature 

 for a given region is consequently small. The region which has 68 

 F. 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 lati- 

 tudes of the state of New York, follows the same course nearly as the 



* The word isocrymal here introduced is from the Greek utog, equal, and xfufAo?, 

 extreme cold, and applies with sufficient precision to the line 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 science. 



