78 ISOLATION AS A FACTOR 



form of isolation or segregation. By the effect of 

 some form of barrier the members of one group 

 are prevented from interbreeding with those of 

 another minor group or with the mass of the 

 species. As a result, from difference of parent- 

 age, or difference in selection, or from difference 

 in the trend of development, whatever its cause, 

 local pecuharities arise. " Migration," says Dr. 

 Coues (and by this he means the shifting of hab- 

 itation) , " holds species true ; localization lets 

 them slip " ; or, rather, locahzation leaves them in 

 differing conditions in the general process of av- 

 eraging up the mass of the species. The pecuh- 

 arities of the parents in an isolated group become 

 intensified by in-breeding. These peculiarities 

 become modified in some continuous direction by 

 the selection induced by the characteristics of 

 the local environment. They may possibly be 

 changed, as some have imagined, in one way or 

 another, by germinal reactions induced by impact 

 of environment. It may be that change of envi- 

 ronment sometimes excites germinal variation. 

 In any event, a new form is sooner or later inev- 

 itable if the segregation is complete. This new 

 form is never coincident in range with the parent 

 species, nor with any other closely cognate or 

 germinate form. Neither is it likely to be found 

 in some remote part of the earth. The details 

 of its distribution will be determined by the na- 

 ture of the organism and by its relation to its 

 environment. The struggle for existence is a 



