154 On an Isothermal Oceanic Chart, illustrating 
month in this climate, neither is the winter the coldest season in 
all parts of the globe, especially near the equator. On this ae- 
count, 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 
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 favor 
the growth of species that would be altogether cut off by the cold 
winters where the extremes are more intense. On this account, 
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 region is consequently small. The region which 
has 68° EF’. for its winter temperature, has about 80° for the hot- 
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. 
_4n fresh-water streams, the waters, where not frozen, do not 
sink lower than the colder oceans, reaching at most but a few de- 
* The word isocrymal here introduced is from the Greek wsos, equal, and x, p08, 
extreme cold, and applies with sufficient precision to the lines for which it ag 
; al lines, as these follow the mean winter temperature ; 
and to use this term in the ease before us, would be giving the word a signification 
best does aot ete to it, and making confusion in the science, 
oreover, the greatest range for all oceans is but 62° of Fahrenheit, the highe: 
being 88°, and the lowest 26°; while the temperature of mosphere of the 
has a range exceeding 150°, msi | 
. 
