328 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



and the atmospheric dust, composed in great part of infusorial remains, 

 form materials which add to the deposits of inorganic substances. 



The strength or durability of strata formed of the remains of marine 

 animals depends upon certain exterior conditions which are very vari- 

 able. Thus, the structure of their sides, their petrific character, and their 

 contour, more or less broken ; the nature of the bottom of the sea, which 

 may be muddy, sandy, gravelly, and filled with broken shells and corals ; 

 the tides, the difference between the high and low water ; oceanic cur- 

 rents, transporting the germs to a distance ; climate ; the composition oj 

 the waters, their degree of saltness, and particularly their depth, all con- 

 tribute to complicate the character of a marine fauna, and determine, 

 more or less, the abundance and the variety of forms of a particular 

 region.* E. Forbes has established certain laws upon the relation 

 which exists between the bathymetrical and geographical extension of 

 the marine animals. He has shown that the extension of a species in a 

 vertical direction corresponds to its horizontal or geographical distribu- 

 tion, and, counversely, that the more a species extends in a vertical di- 

 rection, or, in other words, the more it is found at different depths upon 

 the same coast, the more it is diffused over large surfaces. 



The Mollusca rarely live at a depth of more than five hundred yards. 

 Now, for animals which can live only a hundred yards below the surface 

 of the water, all depth beyond this is an insurmountable obstacle to 

 their extension. This fact may explain why on two sides of a cape are 

 often found two almost completely distinct faunas. It has also been 

 observed that northern species, on approaching the equator, seek a 

 depth of water where the temperature is equal to that from which they 

 came. The reverse takes place with tropical species. The result is 

 that the geographical extent of a species must be greater in proportion 

 as it can live at a greater depth. As the molluscal animals do not 

 repeat themselves on the two sides of the equator, although the type 

 may be the same, it must be admitted that the equator is equally an 

 obstacle to the distribution of the Mollusca. t 



A change in the saltness of the water is also an obstacle to the exist- 

 ence of marine types. In the lagunes and salt marshes near the mouths 

 of rivers, Mollusca are found, indicating by their presence every degree 

 of saltness, from sea-water to fresh. If the degree of saltness is changed, 

 the primitive types are replaced by others better adapted to the me- 

 dium. It has been often asked whether an acclimatation of the marine 

 types is not possible in a medium where the change in the saltness takes 

 place very slowly. We may answer in the affirmative if we consider the 

 acclimatation of the Phoca of Lake Baikal which formerly was a part of 

 a vast sea, and also if we take into account the persistence of the lower 

 animals which have endured through all ages, and still form the most 

 numerous inhabitants of our globe, while the existence of the superior 



* D'Archiac, op. cit., t. ii, pp. 190-194. 

 t D'Archiac, op. cit., t. ii, p. 2ZQ>. 



