382 DIFFICULTIES AND IMPORTANCE OF THE CONCEPT 



situations since there is great diversity in the forms of asexuality. 

 Where sexuality is abandoned only temporarily, as in the cases 

 of seasonal parthenogenesis in Daphnia and aphids, there is no 

 problem. It is customary and biologically sound to consider such 

 temporary clones as portions of the total gene pool of a species. 

 Nor is there any major difficulty where sexuality is lost in a 

 single species of a genus or in a number of lines which can be 

 clearly traced back to a common ancestor and where the mor- 

 phological differences are still slight enough to justify combining 

 these "microspecies" into a single collective species (Mayr, 1951). 

 By far more difficult is the situation in many microorganisms 

 where sexual reproduction or its genetic equivalents are totally 

 absent in large groups or at best highly sporadic. To include all 

 descendants of a common ancestor in a single collective species 

 is also impossible in groups like the bdelloid rotifers which, ap- 

 parently without sexual reproduction, have grown to an order 

 with four families, some twenty genera, and several hundred 

 "species" all reproducing partheuogenetically and possibly all 

 descendants from a single ancestor. If such a group were a 

 complete morphological continuum, any attempt to break it up 

 into species would be doomed to failure. Curiously enough there 

 seem to be a number of discontinuities which make taxonomic 

 subdivision possible. The most reasonable explanation of this 

 phenomenon is that the existing types are the survivors among 

 a great number of produced forms, that the surviving types are 

 clustered around a limited number of adaptive peaks, and that 

 ecological factors have given the former continuum a taxonomic 

 structure. Each adaptive peak is occupied by a different "kind" 

 of organism, and it is legitimate to call each of these clusters of 

 biotypes a species. 



The large list of difficulties which the application of the species 

 concept faces may seem to confirm the opinion of those who 

 consider the species as something purely subjective and arbitrary. 

 To counterbalance this impression it must be emphasized (a) 

 that none of these difficulties of application invalidates the three 

 basic concepts of the species; (b) that these difficulties are in- 



