1044 PRINCIPLES OF STRATIGRAPHY 



genetic series of molluscs occurred side by side, and the same is 

 also true to some extent of the corresponding molluscan fauna of 

 the Gulf States of America. 



Faunal Groups. The fauna of any area may be considered 

 as belonging to one or more of the following groups : endemic spe- 

 cies, immigrants and erratics, and relicts. Endemic species are 

 those which originated in the locality in which they are found. 

 Immigrants have invaded the region, and erratics have been car- 

 ried there accidentally, and both iiave estal)lished themselves in the 

 new region. Relicts are remnants, in favored places, of a. once 

 widely distributed fauna, wl?ich, by the breaking up of the area 

 which they occupied, became resolved into a number of local rem- 

 nants, which remain separated. 



Factors Governing Dispersal and Migration. In all cases 

 when considering the laws which govern the. distribution of or- 

 ganisms it is necessary to consider two phases of the subject: 

 first, the chemical, physical, and organic characters of the localities ; 

 and, second, the nature and habits of the organisms — their bionomic 

 characteristics. If the two are harmonious in a given case, it 

 is evident that the locality considered can be inhabited by the or- 

 ganism in question. Under chemical characters we must consider 

 the composition of the medium and its variation ;" and under physi- 

 cal characters we comprise climate and topography of the sub- 

 stratum ; while under organic characteristics we include the presence 

 of suitable food in sufficient quantity, and absence or paucity of 

 competing organisms. 



Inorganic Factors. The Medium. 



Composition of Medium. Under this heading we include the 

 salinity of the sea ; the inclusion of air and other gases in the 

 water, and of water and carbon dioxide in the air. The salinity of 

 the sea is its most characteristic bionomic feature. On the whole, 

 salinity is a pretty constant factor, varying but slightly in surface ex- 

 tent or in depth in the open sea. Along the continental margins, 

 however, in the bays and marginal seas, a great variation is observa- 

 ble. The question of variation has already been discussed at length 

 (Chapter IV), but a few salient points may here be repeated. 

 Thus in the Red Sea, where the supply of water is scanty, and the 

 evaporation great, the salt constituent is 4.3 per cent, or 43 permille ; 

 while in the Baltic, where the supply of fresh water is abundant and 

 evaporation is small, the surface-salinity is very low, averaging 7 

 permille. The decrease in salinity eastward is very striking; thus it 



