M. Alcide d'Orbigny on Living and Fossil Molluscs. 59 



more than any other agents, is the true basis of the geogra- 

 phical distribution of the animals of the high seas. We may 

 add, that they are found to be more complicated in their 

 forms, and more numerous in species, the nearer we approach 

 the warm regions. The pteropods, although more indifferent 

 as to temperature, have afforded me the same general re- 

 sults,* with respect to their geographical distribution in the 

 oceans. 



" The investigations which I have in like manner under- 

 taken, although much more difficult, in order to become ac- 

 quainted with the laws which regulate the geographical dis- 

 tribution of the molluscs of sea-coasts, have led me to curious 

 results. t I have ascertained, for example, the action of 

 three different influences, — currents, temperature, and the 

 orographical configuration of coasts. 



""We thus perceive, that if currents, by their long-con- 

 tinued action, have a tendency to spread the molluscs of 

 coasts beyond their natural limits of latitude, when they carry 

 them to a distance from a continent, or round a cape ad- 

 vanced in the direction of the pole, — or when they suddenly 

 leave the coasts under the w^arm regions, we must ascribe 

 to them, on the other hand, the isolation and establishment 

 of local faunas. 



" I have likewise ascertained that, notwithstanding the 

 active influence of currents, the passive action of heat is 

 everywhere felt in a very marked manner, by forming col- 

 lections of species in more or less restricted limits of latitude. 



" The orographical configuration of the coasts of oceans, 

 by ofi'ering conditions of existence more or less favourable to 

 littoral molluscs, according to their genera, exercises also 

 immense influence on the zoological composition of the faunas 

 which inhabit them. 



" From the combined effect of these three kinds of in- 

 fluences we may infer, with certainty, that the laws which 



* Memoir read to the Academy of Sciences in 1835, and inserted in tlie 

 molluscs of my Voyage dans i' Amerique Meridionale, p. 68. 



t 8ee my Memoir, laid before the Academy of Sciences in November 1844, 

 and printed in 1845 in the Annaks det Science* Nalarelles. 



