THE ORIGIN OF SPECIES 



ferred to the other. Heiser has found that each of these species of sun- 

 flower varies in the direction of the other. While this could be accounted 

 for by parallel mutation in the two species, he believes that introgression 

 is a much more likely explanation. 



It was noted above that hybridization of these species is particularly 

 likely to occur in areas disturbed by man. Anderson, who has made ex- 

 tensive studies on introgression in plants, believes that this may be a nec- 

 essary requirement for the survival of hybrid swarms. The reason is that 

 the hybrids are likely to require habitats intermediate between those to 

 which the parent species are adapted. He therefore speaks of the "hybridi- 

 zation of tlie habitat." As man disturbs nature with his manifold activities, 

 such "hybrid habitats" are likely to be formed, with the result that he 

 facilitates the exchange of genetic variability between related species, and 

 hence increases the range of types from which selection can choose the 

 most advantageous. 



Introgressive hybridization has not been widely investigated among ani- 

 mals, yet Hubbs has stated that his studies on fishes indicate that this 

 process is also active in the evolution of the fishes, and examples have also 

 been published for birds and other groups. 



REFERENCES 



Anderson, E., 1949. "Introgressive Hybridization," John Wiley & Sons, Inc., New 

 York, N.Y. Still the major work on this subject. 



DoBZHANSKY, T., 1951. "Genetics and tlie Origin of Species," 3rd Ed., Columbia Uni- 

 versity Press, New York, N.Y. ( Dice and Blossom, Spieth. ) 



GoLDSCHMiDT, R. B., 1940. "The Material Basis of Evolution," Yale University Press, 

 New Haven, Conn. (Turel. ) 



Mayr, E., 1942. "Systematics and the Origin of Species," Columbia University Press, 

 New York, N.Y. 



(The above three books, all of which have been introduced in connection with 

 preceding chapters, all make important contributions to the subject of isolation. ) 



292 



