522 Cytogenetics and Evolution 



Mechanical and Psychological Isolation. Ecological and sea- 

 sonal isolations are mechanisms that prevent two species from 

 producing hybrids by keeping them apart either in space or in 

 time as the result of forces partly within the species. There are 

 two types of isolation mechanisms that keep two species from 

 mating, even though they are close enough to one another to 

 mate and even though they are both capable of reproduction at 

 the same time. One of these is mechanical isolation. 



It has been found that in some families of animals considerable 

 differences are present in the external genital organs in different 

 species. These differences are so marked in some forms that they 

 have been used by taxonomists in distinguishing different species 

 in some insects, in spiders, in mollusks, and in some fish and 

 mammals. Because of these differences, the theory has been 

 advanced upon a number of occasions that in forms with such 

 complicated genital organs only the male and female of the same 

 species are able to mate. There is only a slight amount of evi- 

 dence to support this theory, and there are numerous observa- 

 tions of successful mating between species that are not closely 

 related. More factual data are needed to clarify this point, but 

 it appears to be the view of Dobzhansky, Kinsey, and other 

 zoologists that the importance of this type of isolating mechanism 

 has been greatly exaggerated. 



Another mechanism that prevents mating even though the spe- 

 cies are not separated in either time or space has been called 

 psychological isolation or sexual isolation. In this type of iso- 

 lation there are no morphological differences in the sex organs, 

 but there are differences in the patterns of behavior that precede 

 mating. 



Some interesting examples have been pointed out by Dice for 

 mice of the genus Peromyscus. In Glacier National Park, Mon- 

 tana, Peromyscus maniculatus artemisiae, a subspecies that in- 

 habits the forests and P. m. osgoodi, a grassland subspecies, are 

 found together but do not interbreed. Ecological isolation ac- 

 counts for some of the failure of interbreeding, but will not ac- 

 count for all of it. In a number of places where the two habitats 

 meet, both subspecies live together but do not interbreed. This 

 failure of mating appears to result from psychological differences 

 between the two subspecies. A similar failure to interbreed has 

 been found in regions of the Dismal Swamp of Virginia, in north- 



