EFFECT OF INTERRUPTED MIGRATIONS 287 



more complex, the original strains are themselves composites of complex 

 origin, and the present habitat of the nearest living representatives of 

 these strains is not their original home. 



Effect of interrupted Migrations 



The problem has thus far been treated on the assumption of free op- 

 portunity for dispersal and continuous environmental pressure, produc- 

 ing a continuous stream of migration. But in fact we must expect peri- 

 odic interruption of greater or less duration in this movement, due either 

 to barriers interposed or to temporary cessation or reversal of the en- 

 vironmental pressure, of whatever nature this may be. These interrup- 

 tions, if short, will produce nodes ; if longer, actual breaks in the stream 

 as it moves forward in time and space. The nodes in the further progress 

 of the stream will tend to disappear; the actual breaks involving infer- 

 tility may, if sufficiently great, be permanent. In a region far from, the 

 center of dispersal the arriving stream will preserve completely or par- 

 tially all the nodes and breaks due to interruptions at former stages of 

 its progress, the nodes tending to disappearance, the well marked breaks 

 to sharper accentuation. Hence the farther away we are from the center 

 of dispersal the more the race will be broken up into sharply differen- 

 tiated species or genera, and the less in proportion will there be of nodes 

 or imperfectly differentiated and anastomosing groups. 



Effect of Eemoteness from Dispersal Center 



Furthermore, a race remote from its center of dispersal will have 

 passed through many different local environments on its way, and each 

 of these, especially during interruptions of the migration movement, will 

 have tended to impress on its members a local adaptive radiation, which 

 will increase and intensify the diversity of the resultant in its further 

 course. The influence of the typical evolution of the race proceeding in 

 its center of dispersal will be more remote; that of the various divergent 

 adaptations passed through will be more dominant, as the race is more 

 remote from its original home. 



Near the center of dispersal, therefore, we should expect to find the 

 specific and generic groups of a family difficult to distinguish, intergrad- 

 ing a great deal and held more strictly to type. Far from the center o( 

 dispersal we should expect more sharply differentiated species or genera, 

 more diversity of type, and more aberrant forms. 



