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



the surface of the globe a distinct fauna, but identical in its 

 composition ; thus the silurian, devonian, and carboniferous 

 stages, the triasic, Jurassic, cretaceous and diluvian forma- 

 tions, appear to be the same over the whole earth,* and there 

 preserve the same generic forms, along with the same palseon- 

 tological /ac«e*. 



" Not only is there the same fades and the same generic 

 forms in the lost fauna of the whole globe, but there are also 

 some identical species, common everywhere, which prove how 

 completely they are contemporaneous. 



" This contemporaneous existence which we remark at im- 

 mense distances from the first period of animalization,t and 

 even up to the time when the lower cretaceous strata were 

 deposited, + seems to depend on a uniform temperature and 

 the shallowness of seas ; indeed, such conditions would allow 

 these beings not only to enjoy everywhere the influence of 

 the exterior light, a circumstance indispensable to their ex- 

 istence, but also to propagate and spread themselves without 

 obstruction from one place to another. But this state of 

 things could not be continued after the influence of latitude, 

 and, consequently, the inequality of temperature caused by 

 the cooling of the earth, on the one hand, and, on the other, 

 the elevation of the eai'th as well as the great depths of the 

 ocean produced barriers which the sedentary zoology of the 

 coasts could not pass beyond. We must, therefore, suppose 

 that the uniformity in the distribution of the earliest beings 

 on the globe is owing as much to the equality of tempera- 

 ture determined by the central heat, as by the shallowness 

 of the seas ; while the separation of faunas by basins of 

 greater or less extent, arises, as we approach the existing 

 period, from the cooling of the earth, the limits of latitude, 

 ten-estrial barriers caused by continents, and marine barriers 



* I have found it to be the case at least in regard to America and Europe. — 

 Voyage dans PAmerique Meridionale, vol. iii. ; Palcontologie, p. 175. 



t The Productus, the Spirifer, and the species of other genera are found 

 simultaneously iu Europe and America. 



% See my Possiles de Colombie, 1842, where many species are identical in 

 America and Europe. 



