THE SPECIES PROBLEM FROM THE 

 VIEWPOINT OF A PHYSIOLOGIST 



C. LADD PROSSER: physiology department, university 



OF ILLINOIS, URBANA, ILLINOIS 



The principal task of physiologists is to describe life functions 

 as they occur in well-known organisms; physiologists are less 

 concerned with how organisms came to be than with how they 

 now are. Comparative physiologists attempt to ascertain the 

 ways in which different organisms solve their life problems. By 

 using kind of organism as one experimental variable when study- 

 ing a function, the comparative physiologist can arrive at different 

 kinds of generalizations from those obtained by the more in- 

 tensive study of single organisms. As part of the effort to describe 

 life processes, some physiologists investigate adaptations to en- 

 vironmental stresses; these adaptations constitute the functional 

 bases for animal distribution, and consideration of them leads 

 inevitably to the species problem. Success of a species at its en- 

 vironmental limits depends on adaptive capacity in structure and 

 function, but the morphology which is related to such essential 

 function is often at quite a different level from that used by the 

 systematist; it may be molecular morphology. At the same time, 

 the morphological characters of the systematist's key, whether 

 adaptive or not, are generally assumed to be associated geneti- 

 cally with physiological characters; the genetic basis for adaptive 

 physiological characters is usually multifactor and complex. 



Most of the physiologists and biochemists who have considered 

 phylogeny have compared higher taxonomic units — phyla and 

 classes — and they have generally supported the conclusions 

 reached previously by comparative embryologists and classical 

 phylogenists. Relatively few physiological comparisons have 

 been made of closely related species or of populations living at 



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