E. MAYR 387 



pelagic larval stages. Such organisms must be capable of coping 

 with the water conditions in the area where they settle. It would 

 be a mistake, however, to conclude from this special case that 

 all infraspecific population differences of physiological characters 

 are nongenetic. There is not only the excellent work of the 

 Stanford group (Clausen, Hiesey, and Keck) and their associates, 

 which clearly establishes the genetic nature of physiological 

 differences between local races of plants, and similar results have 

 been obtained elsewhere, but also a large body of fact demon- 

 strating geographic variation of physiological characters in ter- 

 restrial animals. The ultimate differences between species are in 

 part built up from such population differences. However, a dis- 

 tinction must be made between purely ecotypic local adaptation 

 to purely local conditions and a more general physiological 

 divergence which is almost always found in isolated populations 

 (Mayr, 1956). Physiological differentiation sometimes proceeds 

 more rapidly than morphological change, and this is the explana- 

 tion for the occurrence of sibling species in many groups. 



The study of physiological adaptations and limitations is still 

 very much at the beginning. This seems to be one of the most 

 promising branches of comparative physiology. A study of the 

 physiological differences between species, particularly aquatic 

 species, with respect to rates of development, temperature adap- 

 tation, and temperature tolerance has already yielded most in- 

 teresting findings. 



These are only a few indications of newly developing branches 

 of biology specifically dealing with the species level. They refer 

 to the development I had in mind when I said that we were wit- 

 nessing at the present time the growth of a "science of species." 



Conclusion 



The discussions of this symposium show clearly that species 

 are still a stimulating subject and that the species problem still 

 offers a challenge. There are many difficulties in applying the 

 species concept to the vast variety of discontinuities found in 

 organic nature. Yet many biological phenomena would make no 



