V. GRANT 75 



trast to the situation which the zoologist finds in higher animals 

 in which clearly circumscribed species are the rule and poorly 

 defined ones the exception. 



A survey of some typical plant genera in which systematic 

 difficulties are encountered shows that the species problem is due 

 to a variety of causes. ( 1 ) Where species originate from races by 

 a continuous and gradual process of evolution, some populations 

 at any given instant will be in a halfway stage between race and 

 species. (2) Some populations which must be acknowledged as 

 species on the basis of reproductive isolation do not happen to 

 possess easily visible distinguishing characters. (3) The isolation 

 of species may not be complete with the result that some inter- 

 mediate populations of hybrid origin may arise. (4) Plants often 

 revert to asexual means of propagation in which case the very 

 basis of the organization of populations into species, namely in- 

 terbreeding and exchange of genes internally and reproductive 

 isolation externally, is lost. The most important single cause of a 

 species problem in plants is natural hybridization. 



How does the biological species concept stand up in the light 

 of the various difficulties of application listed above? The objec- 

 tive reality of the species as a biological unit is not impaired by 

 the gradual formation of species or by the existence of cryptic 

 species. The loss of sexuality, on the other hand, simply means 

 that true biological species do not exist in certain plant groups. 

 The effects of natural hybridization on the objective reality of 

 the species, finally, cannot be summarized in categorical terms, 

 since the discreteness of the species unit is a relative matter in a 

 hybridizing complex. 



Inasmuch as the species is defined as an interbreeding popula- 

 tion isolated from other populations by reproductive barriers, 

 natural hybridization is a factor opposed to the integrity of the 

 species. If the hybridization is very limited in extent, its effects 

 may be overlooked in taxonomic practice. But if the hybridiza- 

 tion continues on an extensive enough scale, the result may be 

 the breakdown of the original species. Between these extreme 

 conditions there is a wide range of intermediate stages in species 

 disintegration due to hybridization. Where the process of hybridi- 



