LECTURE XXXII. 

 ISOLATION AS A FACTOR IN EVOLUTION. 



Barriers separating forms permit them to differen- 

 tiate. " Migration holds species true, localization lets 

 them slip." Compare the local variations in forms 

 in California with those in Mississippi Valley. Much 

 greater number of varieties, fewer forms. 



Compare races and dialects in Europe with those in 

 America. 



Physiological segregation. Peculiar forms so sepa- 

 rated as to despise each other. Tendency towards 

 sterility. 



Race prejudices ; deer ; men. 



Hybridism. Its value as a test of species slight. 



1. Possibility of amphimixis. 



2. Possibility of fertilized egg developing. 



3. Possibility of resultant animal having fertile 

 germ cells. 



