352 MANUAL OF THE MOLLUSCA. 



rated by lofty mountains, deserts, seas, and climates ; whilst the seas 

 are divided by continents and influenced by the physical character of coast- 

 lines, by climates and currents. These " natural barriers " as they were 

 called by BuflFon, retard or altogether prevent the migrations of species in 

 particular directions. 



Influence of Climate. — Diversity of climate has been the popular explana- 

 tion of most of the phenomena of geographical distribution, because it is 

 so well-lmown that some species require a tropical amount of warmth, 

 whilst others can endure a great variety of temperature, and some only thrive 

 amidst the rigours of the arctic regions. The character of the vegeta- 

 tion of the zones of latitude has been sketched by Baron Ilumboldt ; Fabricius 

 and Latreille have divided the world into climatal Insect-provinces; and 

 Prof. E. Forbes has constructed a map of Jthe liomoiozoic belts or zones of 

 marine life. To all these the remark of Mr. Kirby is applicable — that any 

 division of the globe into pro\dnces, by means of equivalent parallels and 

 meridians, wears the appearance of an artificial and arbitrary system, rather 

 than of one according to nature. Prof. Forbes has been careful to point out 

 that although the " Faunas of regions under similar physical conditions bear 

 a striking resemblance to each other " — this resemblance is produced, " not 

 by identity of species, or even of genera, but by representation." (p. 56). 



Origin of the Natural History Provinces. — Mr. Kirby appears to have 

 been the first to recognize the truth that physical conditions were not the 

 primary causes of the zoological provinces, wliich he " regarded as fixed by 

 the will of the Creator, rather than as regulated by isothermal lines."* Mr. 

 Swaiuson also has shown that the "circumstances connected with temperature, 

 food, situation and foes, are totally insufiicient to account for the phenomena 

 of animal geography," which he attributes to the operation of unknown laws.f 



The most important contribution towards a knowledge of these "unknown 

 laws " has been made by Prof. E. Forbes, who was perhaps the first naturalist 

 ever in a position to avail himself of the great storehouse of facts accumulated 

 by geologists, respecting the distribution of organic life in "the former 

 world." This subject will be referred to again in connection wath the subject 

 of Fossil Shells ; meanwhile it may be stated, that according to this evidence, 

 the Faunas of the Provinces are of various ages, and that their origin is con- 

 nected with former (often very remote) geological changes, and a difl"erent 

 distribution of land and water over the surface of the globe. 



MARINE PROTINCES. 



Amongst the genera of marine shells, there are some which have been 

 considered particularly indicative of climate. From the Arctic list the foUow- 



* Introduction to Entomology. 



t Treatise on Geography and Classili'-^ation o*" Animals, Lardner's Cabinet CycJo- 

 peeJia. 



