V. GRANT 77 



the animal body probably reflects an equally complex and deli- 

 cately balanced ensemble of polygenes, whereas the far more 

 simply organized plant body is the resultant of a simpler and 

 more loosely integrated gene system. Foreign genes will conse- 

 quently be much less likely to fit harmoniously into the highly 

 integrated genotype of an animal than into the more loosely 

 organized genotype of a plant. Natural selection will accordingly 

 eliminate the products of interspecific hybridization, or prevent 

 the hybridization from occurring by the erection of breeding 

 barriers, to a greater extent in animals than in plants. 



Acknowledgments. The manuscript was critically reviewed 

 by Dr. Jay M. Savage, of the University of Southern Cali- 

 fornia, to whom the author is indebted for numerous helpful 

 suggestions. Mr. Howard Latimer of the Rancho Santa Ana 

 Botanic Garden also made constructive suggestions with regard 

 to one section of the manuscript. The problems of classification 

 in certain difficult western American genera of plants were dis- 

 cussed with Dr. Philip A. Munz of the Rancho Santa Ana Bo- 

 tanic Garden. The interest and advice of these colleagues is 

 gratefully acknowledged. Finally, the deliberations in Atlanta 

 brought out several new points, which were taken into considera- 

 tion in the preparation of the final manuscript. 



The work on the Cobwebby Gilias, incorporated into the dis- 

 cussion of the effects of hybridization on species distinctness, 

 was accomplished with the aid of a research grant from the Na- 

 tional Science Foundation, which is likewise thankfully ac- 

 knowledged. 



REFERENCES 



Anderson, E. 1953. Introgressive hybridization. Biol. Revs. Cambridge 

 Phil. Soc, 28, 280-307^ 



Anderson, E., and G. L. Stebbins, Jr. 1954. Hybridization as an evolu- 

 tionary stimulus. Evolution, 8, 378-88. 



Baker, H. G. 1951. Hybridization and natural gene-flow between 

 higher plants. Biol. Revs. Cambridge Phil. Soc., 26, 302-37. 



