1454 



CRUSTACEA. 



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 subdivision of the Regions into many sub- 

 ordinate Districts. In oceanic productions, depth and kind of bottom 

 have an important bearing : but there is no occasion to consider the 

 moisture or dryness of the climate; and the influence of the other 

 peculiarities of region mentioned is much less potent than with conti- 

 nental life. 



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

 have been gathered, as regards the North Atlantic, from the iso- 

 thermal 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 Meteoro- 

 logical volume of Captain Wilkes of the Exploring Expedition Reports, 

 which embraces observations in all the oceans and valuable deductions 

 therefrom ; also, the records of other travellers, as Humboldt, Duper- 

 rey of the Coquille, D'Urville of the Astrolabe, Kotzebue, Beechey, 

 Fitzroy, Vaillant of the Bonite, Ross in his Antarctic Voyage, toge- 

 ther with such isolated tables as have been met with in different 

 Journals. The lines we have laid down, are not however, those of 

 any chart previously constructed, for the reason stated, that they 

 mark the positions where a given temperature is the mean of the 

 coldest month (or coldest thirty consecutive days) of the year, instead 

 of those where this temperature is the mean annual or monthly heat ; 

 and hence, the apparent discrepancies, which may be observed, on 

 comparing it with isothermal charts. 



The isocrymal lines adopted for the chart are those of 80°, 74°, 

 68°, 62°, 56°, 50°, 44°, and 35° of Fahrenheit. They diminish by 6°, 

 excepting the last, which is 9° less than 44°. 



In adopting these lines in preference to those of other degrees of 

 temperature, we have been guided, in the first place, by the great fact, 

 that the isocryme of 68° is the boundary line of the coral-reef seas, as 



