72 THE PLANT SPECIES 



tion, similarly, affects the species problem most markedly where 

 it is associated with hybridity, as is the case in both the agamic 

 complex and the clonal complex. Even self-pollination, which 

 under normal conditions is compatible with good species divi- 

 sions, can if associated with hybridity and other peculiarities of 

 the genetic system cause a disintegration of true species, as illus- 

 trated by the heterogamic complex of the Oenothera hookeri- 

 biennis group. 



Natural hybridization is thus directly or indirectly involved in 

 nearly all the major sources of a species problem in plants. The 

 conclusion is inescapable that this process is the most important 

 single factor in the breakdown and indistinctness of plant species. 

 The conclusion that plant species are more affected by hybridiza- 

 tion than animal species agrees well with the observed differences 

 in the average clarity of species boundaries in the two kingdoms. 



Physio-Genetic Nature of the Species 



Among evolutionists there has been some speculation as to the 

 reasons for the generally higher level of reproductive isolation in 

 animals than in plants. Ethological isolating mechanisms prob- 

 ably account for a part of the difference. Specific behavior pat- 

 terns which stimulate conspecific matings but inhibit interspecific 

 matin gs constitute an important element in the complement of 

 species-separating barriers in most animals. In higher plants such 

 isolating mechanisms, while not entirely wanting, are certainly 

 represented in a much weakened form. The sterility barriers also 

 seem to be more nearly absolute in many animal groups than in 

 plants. This may be a consequence of the apparent prevalence 

 of genie sterility in animals and of chromosomal sterility in plants 

 (Dobzhansky, 1951), as well as of the greater absolute fecundity 

 of long-lived plants (Stebbins, 1950). 



Explanations such as these, however, merely touch the surface 

 of the problem. If animal species are in general less contaminated 

 by hybridization than plant species, the ultimate cause must be 

 sought not in the strength of the isolating mechanisms but in 

 the evolutionary forces which build up those isolating mecha- 

 nisms. A possible explanation may be that natural selection favors 



