The Principles of Palaeontology. 203 



complete or incomplete, is then, in different degrees, an indirect 

 cause of the specialization of marine faunas. But it is evident 

 that it is also a direct obstacle to the diffusion of a fauna already- 

 specialized through the action of other factors. 



The currents have also a double effect which opposes that of 

 barriers ; on the one hand every current established between two 

 regions of different climates will result in producing a sort of 

 mixture between the bodies of water influenced by different 

 temperatures, and it will affect ^also the aerial climate ; the 

 example of the Gulf Stream is too well known to make it neces- 

 sary to emphasize this point. But oa the other hand these cur- 

 rents will bring with them the larvae of various animals which 

 will penetrate more or less into a zone where they were before 

 unknown. This migration of faunas may occur without any 

 very notable change in the general climate, provided the current 

 does not put in motion any considerable masses of water ; thus 

 the current of Gibraltar introduced into the Mediterranean dur- 

 ing the Pliocene epoch, forms belonging to the Atlantic, and 

 those forms have remained in the Mediterranean, although the 

 mean temperature there is now much higher than that of the 

 Atlantic. 



It is easy to understand that in studying geologic eras it is 

 very difficult to form an exact idea of the influences of barriers 

 and currents. When we have established the presence of two dis- 

 tinct faunal zones which display no differential characteristics in 

 the respects mentioned above ; when in the same region we note 

 the appearance of a fauna which before this period existed in 

 other basins, we are sometimes perplexed as to the influence to 

 be assigned to variations of climate, or to barriers and cur- 

 rents. We may succeed sometimes by closely comparing the 

 results furnished by the marine fauna with those afforded by the 

 study of the flora and of the fresh-water, terrestrial or even 

 serial fauna. 



The problem of the determination of climates at ancient 

 epochs through the data of Palaeontology is by no means insolu- 

 ble ; it has been the object of profound and ingenious researches, 

 the results of which already attained deserve our attention for 

 a moment.* 



♦ Neumayr, Erdgeschichte, toI. II. Heer, Le Monde primitif de la Suisse, 



