THROUGH CUMrLATIYE SEGREaATION. 



231 



water, and the action of migratory species upon those that can 

 simply cling). 



8. Geological Segregation, caused by geological changes di- 

 viding the territory occupied by a species into two or more 

 sections. For example, geological subsidence may divide the 

 continuous area occupied by a species into several islands, 

 separated by channels which the creatures in question cannot 

 pass. 



Migration differs from transportation simply in that the former 

 is the direct result of activities in the organism, and the latter 

 of activities in the environment; and though the distribution of 

 every species depends on the combined action of both classes of 

 activities, it is usually easy to determine to which class the 

 carrying power belongs. The qualities of the thistle-down 

 enable it to float in the air, but it is the wind that carries it afar. 



Some degree of Local Segregation exists whenever the members 

 of a species produced in a given area are more likely to interbreed 

 with each other than with those produced in surrounding areas, 

 or whenever extraordinary dispersal plants a colony beyond the 

 range of ordinary dispersal. In other words, when those pro- 

 duced in a given district are more nearly related with each other 

 than with those produced in surrounding districts, there local 

 segregation has existed. 



There is one important respect in which Spatial Segregation 

 differs from all other forms of Environal Segregation, namely, in 

 its ordinary operation it does not depend directly upon diversity 

 in the qualities and powers of the organism. The dispersion of 

 the members of a species would not be prevented if eacli was 

 exactly like every other ; though, of course, if there were no 

 power of variation, separate breeding would have no influence in 

 producing divergence of character. It follows that every species 

 is, or is more or less liable to be, aff'ected by Spatial Segregation ; 

 and it often happens that other forms of Segregation arise 

 through the previous operation of this form ; but as Spatial 

 Segregation prevents organisms from crossing only when sepa- 

 rated in space, it must always be reinforced by other forms 

 of segregation before well-defined species are produced that 

 are capable of occupying the same district without interbreeding. 

 The vast majority of the divergent forms arising through Local 

 Segregation are reintegrated with the surrounding forms, new 

 divergences constantly coming in to take the place of the old ; 



