TAXONOMY OF THE HIGHER PLANTS 1 97 



tinuing emphasis of taxonomists and evolutionists generally is upon the dis- 

 continuities found between different taxa. Separations of relatively similar 

 types are easier by the use of discontinuities than by the use of the correlated 

 characteristics of each type. Hence it is only natural that discontinuity should 

 receive first attention in arriving at a classification. Certainly, the idea of 

 discontinuity between species is firmly fixed in the biologist's mind. Discon- 

 tinuities are of a variety of sorts ranging from spatial or geographical sepa- 

 ration to genetic incompatibility. But it is the latter that has gotten in the 

 way of a full appreciation of interspecific hybridization. The belief is wide- 

 spread that one of the obvious characteristics of a species is that it does not 

 and cannot hybridize and exchange genes to any degree with related species. 

 Yet the number of known cases is steadily increasing where different species 

 do hybridize, and the resulting hybrids show but little indication of incom- 

 patibility between themselves or with their parental species. Even where 

 incompatibility is present, it may not be all or none but a degree of in- 

 compatibility that will permit some gene exchange between species, with im- 

 portant consequences to the species involved. Thus, the introgression of some 

 of the genetic materials of one species into another may occur, in spite of the 

 presence of genetic incompatibility between them. Reproductive isolation in 

 such a case is not complete. 



The real point to be made here is that taxonomic studies dealing with 

 hybridization are very much in the forefront of the field and bid fair to con- 

 tribute sizably to a fuller understanding of the interrelationships of many 

 plant groups and to an understanding of their evolutionary history. We no 

 longer think of an evolutionary pattern for plants in terms of a dendritic, 

 ever-diverging arrangement. Rather, we recognize the numerous possibilities 

 for anastomosing lines, once separated, into an interlocking network. 



GENETICAL VS. ENVIRONMENTALLY CONTROLLED 

 CHARACTERISTICS 



Some of the fundamental tenets of taxonomic botany and of genetics were 

 seriously challenged by the published work of Gaston Bonnier (1920), which 

 received a rather wide hearing among biologists generally. Bonnier claimed to 

 have converted a number of lowland species into different but related high- 

 altitude species by transplanting them from their low-elevation stations to 

 the same habitat with similar and presumably related species at high eleva- 

 tions. The changes said to be induced by the changed environment were pre- 

 sumed to be permanent. Stimulated by Bonnier 's findings, F. E. Clements 

 conducted transplant experiments on Pikes Peak in Colorado and made 

 claims similar to those of Bonnier. These experiments of Bonnier have since 

 been shown to be erratic and unreliable, and the claims of Clements remain 

 unsubstantiated by any reliable subsequent work. The early work of Kerner 



