SPATIAL ISOLATION. 127 



not bring together those of similar endowments, and does not depend 

 on diversity. The dispersion of the members of a species would not be 

 prevented if each 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 more or less liable to be affected by spatial isolation; 

 and it often happens that other forms of isolation arise through the 

 previous operation of this form; but as spatial isolation prevents 

 organisms from crossing only when separated in space, it must always 

 be reinforced by other forms of isolation before well-defined species 

 are produced that are capable of occupying the same district without 

 interbreeding. Many slightly divergent forms arising through local 

 isolation are reintegrated with the surrounding forms, new diverg- 

 ences constantly coming in to take the place of the old ; but if, during 

 its period of local divergence, industrial or chronal isolation is intro- 

 duced, the variety becomes more and more differentiated, and, as one 

 after another the different forms of reflexive segregation arise, it 

 passes into a well-defined species. 



As spatial isolation does not necessarily depend upon diversity in 

 the qualities and powers of the organism, it usually fails of distributing 

 the varieties of a species in different localities according to their 

 differences of endowment. The causes that produce it are primarily 

 separative, not segregative. 



Migration is produced by the natural powers of the organism acting 

 under the guidance of instincts that usually lead a group of indi- 

 viduals, capable of propagating the species, to migrate together; 

 while organisms most dependent on activities in the environment 

 for their distribution are usually distributed in the form of seeds 

 or germs, any one of which is capable of developing into a complete 

 communily. 



The causes of isolation between the different sections and of inte- 

 gration between the members of one section are, therefore, sufficiently 

 clear ; but what are the causes of differences of character in different 

 sections, especially when they are exposed to the same environment ? 

 These causes all come under what I call typal intensification through 

 diversity of success and diversity of survival, 



