E. MAYR 19 



to fall back on the criterion of "degree of difference." It will be 

 one of the tasks of this symposium to investigate to what extent 

 the diversity of animal and plant life permits application of the 

 standard yardstick of the biological species concept. When can 

 it not be applied and for what reasons? What types of difficulties 

 are there? And finally, are there perhaps other basic concepts 

 in addition to the stated ones? By approaching the species prob- 

 lem with new questions and new material, perhaps this sympo- 

 sium can make a contribution to its solution. 



REFERENCES 



Arkell, W. J. 1956. The species concept in paleontology. Systematics 



Assoc. Publ. No. 2, pp. 97-99. 

 Bachmann, H. 1905. Der Speziesbegriff. Verhandl. schweiz. natur- 



forsch. Ges., 87, 161-208. 

 Baer, K. E. von. 1828. Entwickelungs-Geschichte der Thiere. Konigs- 



berg. 

 Besnard, A. F. 1864. Altes und Neues zur Lehre iiber die organische 



Art (Spezies). Ahhandl. zool. mineral. Ver. Regensburg, 9, 1-72. 

 Bessey, C. E. 1908. The taxonomic aspect of the species question. Am. 



Naturalist, 42, 218-24. 

 Brauer, F. 1885. Systematisch-zoologische Studien. Sitzber. Akacl. 



Wiss. Wien, 91 (Abt. 1), 237-413. 

 Britton, N. L. 1908. The taxonomic aspect of the species question. 



Am, Naturalist, 42, 225-42. 

 Burma, B. H. 1949a. The species concept: A semantic review. Evolu- 

 tion, 3, 369-70. 

 Burma, B. H. 1949b. The species concept: Postscriptum. Evolution, 



3, 372-73. 

 Burma, B. H. 1954. Reality, existence, and classification: A discussion 



of the species problem. Madrono, 7, 193-209. 

 Cain, A. J. 1953. Geography, ecology and coexistence in relation to 



the biological definition of the species. Evolution, 7, 76-83. 

 Camp, W. H., and C. L. Gillis. 1943. The structure and origin of spe- 

 cies. Brittonia, 4, 323-85. 

 Darwin, C. 1859. On the Origin of the Species by Means of Natural 



Selection. London. 



