364 



conditions of existence hinders or prevents dispersion, for the 

 barriers separating the localil ies wit h similar, or but slightly 

 different, conditions may be insurmountable. Under bar- 

 riers to migration we may place topographical barriers 

 first, especially the northward and southward stretching 

 continents, as already pointed out. But topographical 

 barriers are not the only ones, nor in many eases the most 

 important. Differences in temperature, character and direc- 

 tion of ocean currents, improper facies of the sea bottom 

 and insufficient food supply, ns well as hostile species, consti- 

 tute some of the chief barriers to emigration. If by some 

 means or other a barrier is surmounted, and a new colony 



• 



established, this new colony may become more or less 

 isolated, the barrier proving too effective for all but a few 

 individuals. "Migration," says Ortmann ('96, II., p. 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 locality are afterwards practically separated from the 

 original stock remaining in their native country, and thus 

 they may develop separately 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." 



The fauna of any area may be considered as belonging to 

 one or more of the following groups: endemic species, im- 

 migrants, or relicts. Endemic species are those which 

 originated in the localitv in which thev are found. Immi- 

 grants have invaded the region, and established themselves 

 in it. Relicts are remnants, in favored places, of a once 

 widely distributed fauna, which, by the breaking up of the 

 area which thev occupied, became resolved into a number of 

 local remnants, which remain separated. 



