1042 PRINCIPLES OF STRA TKiRAPHY 



appeared at New Haven, Connecticut, in iS8o ( Morse-jj). In 1898 

 Littorina rudis and L. paUiata were still the predominant types on 

 the Long Island coast, but by 1901 L. Uttorca had gained a marked 

 predominance (Balch-3), and it is still rapidly advancing. (Daven- 

 port-ii :i68.) Since the opening of the Suez Canal in 1869 many 

 Mollusca of the Mediterranean have migrated into the Red Sea. 



Barriers to Migration and Dispersal. Under barriers to 

 migration and dispersal we may place topographical barriers first, 

 such as northward and southward stretching continents for marine 

 organisms, desert tracts for fresh water organisms, and great bodies 

 of water for terrestrial organisms. But topographical barriers are 

 not the only ones, nor in many cases the most important. Differ- 

 ences in temperature, character, and direction of winds and ocean 

 currents, improper facies of the substratum, and insufficient food 

 supply, as well as hostile species, constitute some of the chief bar- 

 riers to distribution. If by some means or other a barrier is sur- 

 mounted, and a new colony is established, the new colony may 

 become more or less isolated, the barrier proving too effective for 

 all but a few individuals. "Migration," says Ortmann {2^^:186), 

 "is often slow or only possible under peculiar circumstances, often 

 it is accidental, and only a few individuals can transgress the 

 original limits on rare occasions ; then even migration acts as a 

 means of separation. The few individuals occupying a new local- 

 ity are afterward practically separated from the original stock 

 remaining in their native country, and thus they may develop sep- 

 arately into a different species, even in the case that immigration 

 from the original stock is not altogether impossible, since any rare 

 individuals of the latter reaching the new colony from time to time 

 are soon absorbed by the new form and their characters disappear 

 by the continuous crossing with the modified individuals and by the 

 transforming power of the external conditions." 



Where barriers are numerous, the number of species of a given 

 group of animals or plants is, as a rule, much greater than where 

 barriers are few. Thus in the eastern United States, where impor- 

 tant barriers are wanting, only one true species of chipmunk occurs; 

 while in California, where barriers are numerous, a dozen or more 

 species and sub-species are found (Jordan-25). On the other hand, 

 the fauna and flora of a region with few barriers are likely to show 

 a greater variety of organisms of types which do not interbreed 

 than is found in a region with many barriers, since immigration 

 of many different types is possible into an open region, but pre- 

 vented by the barriers of a closed region. Where separation by 

 barriers has brought about the formation of distinct species, nearly 



