V. GRANT 59 



Darwin, Wallace, and their early followers, the objective exist- 

 ence of species is not compromised by the fact that they come 

 into being by gradual processes of evolution. All the biological 

 units are capable of orderly growth and development in space and 

 time. The duplication of the chromosome strands appears to be 

 achieved in stages, but no one has thought to disqualify the 

 chromosome as an organizational unit because of its gradual 

 mode of reproduction. Similarly, the origin of species in most 

 cases is a process which seems to be completed by stages. During 

 a transitional stage the diverging population is neither one nor 

 two species but is an incompletely separated pair. The circum- 

 stance that the taxonomic hierarchy provides no category for taxa 

 caught midway between races and species will inevitably obscure 

 the biological process nomenclaturally, but should not be allowed 

 to obscure it descriptively. 



The above difficulty is probably in the majority of cases more 

 theoretical than real. Although the evolution of species from races 

 may be continuous, evolution crosses boundary lines. The bound- 

 ary line between race and species, as defined by Dobzhansky 

 ( 1935 ) , is that stage of the evolutionary process when an actually 

 or potentially interbreeding population becomes segregated into 

 two or more reproductively isolated populations. Compared with 

 the duration of life of an average species, the crossing of this 

 boundary line is probably a fairly rapid process. Most populations 

 at any given instant will consequently be found to exist either 

 in the stage of races or in the stage of species, and not in an inter- 

 mediate stage, in so far as their status is affected solely by the 

 gradualness of speciation. 



Natural Hybridization. The difficulties of the species concept 

 which are brought about by natural interspecific hybridization 

 are both theoretical and practical. The difficulty is not apparent 

 and is tacitly overlooked by taxonomists when the hybridization 

 is limited to the rare formation of a few hybrid individuals. It 

 cannot be overlooked in either theory or practice when the hy- 

 brid derivatives establish themselves as an intermediate popula- 

 tion in nature. A mere trickle of genes across an interspecific bar- 

 rier, which is manifested in the production of an occasional 



