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



bution. The courses of the cold-water currents are less evi- 

 dent 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 in- 

 ferred from the remarks already made, and from an exami- 

 nation of the chart itself. 



The distribution of marine life is a subject of far greater 

 simplicity than that of continental life. Besides the influence 

 on the latter of summer temperature in connection with that 

 of the cold seasons, already alluded to, the following ele- 

 ments or conditions have to be considered : the character of 

 the climate, whether wet or dry ; of the surface of the re- 

 gion, whether sandy, fertile, marshy, &c. ; of the vegetation, 

 whether that of dense forests or open pasture-land, &c. ; of 

 the level of the country, whether low or elevated, &c. These 

 and many other considerations come in, to influence the dis- 

 tribution of land species, and lead to a subdivision of the 

 Regions into many subordinate Districts. In oceanic pro- 

 ductions, depth and kind of bottom have an important bear- 

 ing : but there is no occasion to consider the moisture or dry- 

 ness of the climate, and the influence of the other peculiari- 

 ties of region mentioned is much less potent than with con- 

 tinental life. 



We would add here, that the data for the construction of 

 this chart have been gathered, as regards the North Atlan- 

 tic, from the isothermal chart of Lieutenant Maury, in which 

 a vast amount of facts are registered, the result of great 

 labour and study. For the rest of the Atlantic and the other 

 oceans we have employed the meteorological volume of Cap- 

 tain Wilkes of the Exploring Expedition Reports, which em- 

 braces observations in all the oceans, and valuable deduc- 

 tions therefrom ; also the records of other travellers, as 

 Humboldt, Duperey of the Coquille, D'Urville of the Astro- 

 labe, Kotzebue, Beechey, Fitzroy, Vaillant of the Bonite, 

 Ross in his Antarctic Voyage, together with such isolated 

 tables as have been met with in different Journals. The lines 



