CONTINUOUS VERSUS DISCONTINUOUS VARIATION 



the chain are brought together, with Ensatina e. eschscholtzii representing 

 the coastal division, while E. e. croceator and E. e. klauberi represent the 

 interior division. The differences between them are very marked. Stebbins' 

 first paper was based upon 203 specimens of eschscholtzii, 15 of croceator, 

 and 48 of klauberi. Inland and coastal forms were not collected in the 

 same locality, and no intergrades were found. Subsequent collections have 

 enlarged all groups, and several croceator-klauberi intergrades were taken 

 within a few hundred yards of eschscholtzii in the same canyon and the 

 same ecological zone, yet no intergrades with eschscholtzii were found. 

 Evidence was found for neither ecological isolation nor seasonal diflFer- 

 ences in breeding. Hence Stebbins believes that these terminal members 

 of the Ensatina Rassenkreis must meet without breeding, and hence that 

 they would be distinct species if the connecting members of the Rassen- 

 kreis were to become extinct. Scant data are available on their breeding 

 behavior, and it is possible that intergrades may be absent for other rea- 

 sons than intersterility, such as preferential mating, yet there is no evi- 

 dence of this. Genetic studies upon this Rassenkreis, especially upon its 

 terminal members, would be of great interest, although very difiBcult, 

 because they do not breed readily in captivity. This is an exceptionally 

 illustrative Rassenkreis, the facts of which comprise one of the strongest 

 arguments for the neo-Darwinian theory. 



The species question is, then, at once one of the most basic problems 

 of biology and of evolution, and one for which no satisfactory answer is 

 available. It is complicated by the difficulty of comparing sexual and 

 asexual species, by cases in which subspecific or specific status is disputed, 

 and by cryptic or sibling species which show only trivial phenotypic dif- 

 ferences even though they are reproductively isolated, Darwin said that 

 a species in any group is whatever a competent specialist on that group 

 says a species is, and it may well be that species are not the same in 

 different major groups. Discontinuity rather than degree of difference is 

 likely to play the larger role in the achievement of an answer to the 

 species question. Some aspects of the development of discontinuity will 

 therefore be considered in the next chapter. 



REFERENCES 



Huxley, J. S. (Ed.), 1940. "The New Systematics," Oxford University Press. A valu- 

 able collection of essays on many aspects of taxonomy. 



Mayr, E. (Ed.), 1957. "The Species Problem," American Association for the Advance- 

 ment of Science, Washington, D.C. Another valuable collection of essays by many 

 specialists. ( Sonnebom. ) 



Stebbins, R. C, 1949. "Speciation in Salamanders of the Plethodontid Genus Ensa- 

 tina," Univ. Calif. Publ. Zool, 48, 377-526. 



Stebbins, R. C, 1957. "Intraspecific Sympatry in the Lungless Salamander Ensatina 

 eschscholtzii," Evolution, II, 265-270. These two papers by Stebbins form the basis 

 of the above discussion of the Ensatina Rassenkreis. 



269 



