E. MAYR 375 



rectly by the paleontologist Gwynn Thomas ( 1956 ) : "The essen- 

 tial similarity of temporal and spatial variation in fossils also 

 makes the paleontological species concept for practical purposes 

 almost identical with the neontological in fragmentary lineage 

 segments." In either case we are dealing with isolated samples, 

 and in either case we have to base a somewhat arbitrary decision 

 as to species status on a good deal of indirect evidence. Again the 

 difficulty is not with the yardstick but with its application. 



There is a third group of cases where we lack information on 

 the reproductive isolation of individuals, namely, all forms of 

 asexuality. However, in this case more is involved than mere 

 lack of information because here we are not dealing with popu- 

 lations. This then is a much more formidable and more funda- 

 mental obstacle to the application of the species concept, and the 

 discussion of this difficulty shall therefore be postponed to a later 

 section. (See below.) 



Evolutionary Intermediacy 



A species definition postulates at least a discontinuity and, 

 under the most favorable conditions, a triad of characteristics: 

 reproductive isolation, ecological difference, and morphological 

 distinguishability. However, evolution is a gradual process and 

 so is the multiplication of species. As a consequence, there are 

 many instances where a population is on the borderline and has 

 acquired some but not yet all the attributes of a distinct species. 

 The gradual nature of the speciation process raises the following 

 difficulties for the student of multidimensional species. 



Evolutionary Continuity. Species that are widespread in space 

 or time may have terminal populations which satisfy the criteria 

 of distinct species, yet are connected by an unbroken chain of 

 populations. Dr. Moore has described such a situation for con- 

 temporary populations of Rana pipiens; they are indeed not rare 

 in polytypic species. Theoretically this should be the standard 

 condition in paleontology, yet one has difficulty finding such 

 cases in the literature because breaks in the fossil record are suffi- 

 ciently frequent to prevent the piecing together of unbroken 

 lineages. One of the best substantiated ones is that of Micraster 



