the Geographical Distribution of: Marine Species. 155 
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 isotheral lines are of import- 
ance on charts illustrating distribution over the continents, the 
former are pre-eminently important where the geography of 
marine species is to be studie 
The lines of greatest cold are preferable for marine species to 
those of summer heat, because of the fact also that the summer 
range of temperature for thirty degrees 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 parallel 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 re- 
marks already made, and from an examination of the chart itself. 
The distribution of marine life is a subject of far greater sim- 
plicity than that of continental life. Besides the influence on the 
latter of summer temperature in connexion with that of the cold 
Seasons, already alluded to, the following elements or conditions 
have to be considered :—the character of the climate, whether 
wet or dry ;—of the surface of the region, whether sandy, fertile, 
marshy, etc. ;—of the vegetation, whether that of dense forests, 
or open pasture-land, etc. ;—of the level of the country, whether 
low, or elevated, etc. These and many other considerations come 
in, to influence the distribution of land species, and lead to a sub- 
division of the Regions into many subordinate Districts. In 
Oceanic productions, depth and kind of bottom have an important 
bearing: but there is no occasion to consider the moisture or dry- 
ness of the climate ; and the influence of the other peculiarities of 
region mentioned is much less potent than with continental life. 
We would add here, that the data for the construction of this 
chart have been gathered, as regards the North Atlantic, from the 
isothermal chart of Lieutenant Maury, in which a vast amount of 
facts are registered, the result of great labor and study. For the 
