C. L. PROSSER 347 



2. The notion that new characters are retained only if they 

 have adaptive advantage for survival is unrealistic, and there are 

 numerous examples of establishment of neutral morphological 

 and etiological patterns. Neutral characters may be retained 

 if they are linked or pleiotropic with advantageous characters; 

 according to the theory of genetic drift, neutral genes may be- 

 come established in small populations. As more neutral characters 

 are closely scrutinized, they appear to be associated genetically 

 with selective characters and hence they are present in ratios 

 different from those expected by random assortment (Sheppard, 

 1953). For example, human blood types are apparently corre- 

 lated with susceptibility to certain chronic diseases; type A is 

 more frequent and type O less frequent in people with cancer of 

 the stomach than in the general population (Aird et ah, 1953, 

 1954). The real problem of the physiologist is to recognize the 

 critical functional characters. In any case, the characters used 

 by taxonomists to distinguish species are often not adaptive ones. 



3. While the range of a species may be very great, actual 

 breeding populations are often very small. Individuals in migrant 

 species tend to return to delimited areas. This restricts the prob- 

 ability of outbreeding. Territoriality is now recognized not only 

 among vertebrates — mammals, birds, and fish — but in some 

 crustaceans and insects, and it probably exists very widely. The 

 concept of gene flow throughout the range of a species is appar- 

 ently an oversimplification of the actual situation. In general, 

 breeding populations are more susceptible of isolation in fresh 

 water and terrestrial habitats than in marine environments, par- 

 ticularly if in the latter they have pelagic larvae. 



4. Many of the so-called physiological races turn out to be 

 true species. When two populations differ enough to be physio- 

 logically distinct, they usually have developed sufficient differ- 

 ences to become reproductively isolated. The presence of a few 

 hybrids in zones of overlap does not invalidate the basic species 

 distinction, and in the laboratory there may occur interspecific 

 crosses which would be rare or nonexistent in nature. In clines 

 the terminal populations are often clearly different species, but 

 intermediate populations present difficulties for the taxonomist, 



