42 



introduction. "Although workers in other fields have some- 

 times urged that species are mere subjective concepts, to tax- 

 onomists they are usuall}' surprisingly definite entities." He 

 accepts Du Rietz' definition of species as the "smallest natural 

 populations permanently separated from each other by a dis- 

 tinct discontinuity in the series of biotypes." If there is inter- 

 gradation between populations, the separate parts of the 

 species are regarded as subspecies, varieties or forms principally 

 on the basis of their geographical distribution, although they 

 are always separable morphologically. His maps illustrate the 

 distribution of each and in many cases they are quite convinc- 

 ing. In some, on the other hand, there is a very marked overlap 

 (e.g. Lindernia duhia and L. dubia major) and the distinction 

 must have been based primarily on structure. 



The nomenclature of minor groups below the species has 

 always been a problem and unfortunately the existing rules 

 scarcely permit full expression of what we believe to be tax- 

 onomic relationship. The various components of a species are 

 regarded by Pennell as jointly constituting the species. The 

 bibliographically original element is distinguished by the ad- 

 jective typicus. This word, as Pennell carefully explains, is not 

 a part of the name of the plant, but is merely a Latin adjective 

 appended to it, it has no author's name and cannot be referred 

 to any place of publication. There can be either varieties or 

 forms of a subspecies, depending on their importance, but the 

 author refuses to admit forms of a variety. Thus we may find 

 quadrinomials in use, as Lindernia duhia major var. inundata, 

 but a quinquenomial is impossible. This is certainly in pleasing 

 contrast to certain recent European work. 



The 155 maps which accompany almost every species and 

 subspecies are nothing short of remarkable. There is a dot or 

 other symbol for every locality from which Pennell has seen a 

 specimen. They show not only the general range but also the 

 outlying stations; by their density they indicate abundance or 

 conspicuousness; by lacunae among them they often indicate 

 regions where collecting has been neglected. Correlation be- 

 tween geological or geographical features is clearly indicated 

 by superposed lines, marking, for example, the fall-line, the 

 limits of glaciation, and various other features as needed. 



Gray's Manual describes 123 species and named varieties 



