THE CLASSIFICATION OF PLANTS 2125 



all are large enough to be caught in the morphologist's net. Refinements of 

 morphological method have indeed closed the net considerably in recent 

 years and particularly in the discrimination by biometric methods of 

 closely related species living in the same area. Morphological criteria which 

 would once have been dismissed as trivial have been shown to have decisive 

 value within the limited circles of affinity concerned. 



The maximum species, whether conventional or natural, is based not 

 upon individuals but upon populations and is never strictly homogeneous 

 among sexual organisms, or even, though to a lesser extent, among non- 

 sexual or sterile organisms. Every population includes a changeable and 

 changing number of different genotypes, that is to say, genetically differing 

 individuals or groups; which are commonly distinguishable by investigation 

 and often by conventional or " museum " characters. If there exist repro- 

 ductive barriers, other than mere distance, between the population as a 

 whole and related populations, then, notwithstanding its internal diversity, 

 it forms what we have called a maximum species. It is a statistical concept 

 and calls for analysis into subdivisions according to the degree of variability 

 present. These minor groupings, in their turn, also rest on a statistical basis, 

 though a narrower one, and indeed some degree of variability must be 

 accepted in all our groupings if our analysis is to stop short of the individuals 

 themselves. The kind of subdivisions used and the amount and kind of 

 variability accepted will depend on whether we start from the conventional 

 or from the natural standpoint. Let us compare the outcome of the two 

 methods of approach. 



The divisions of the museum species into " sub-species", " variety", 

 " form", etc., are, like the species itself, subjective conceptions which are 

 very difficult to delimit and about their applicability in particular cases 

 systematists often differ. They do not always reflect natural groupings, 

 though they may sometimes do so, and they may conceal, in a subordinate 

 rank, groups which are truly natural species. 



The term " variety " is generally applied by morphological systematists 

 to plants which show a departure from the norm of the species in some 

 character or characters not directly connected with the influence of the 

 environment, nor affecting the diagnostic characters of the species. Plants 

 in which the variation is the direct result of abnormal conditions of growth 

 are generally classed as " forms". The essential point in the use of the 

 term " variety " is that the indicated variation is of stable duration but no 

 questions are asked about its inheritability, genetic constitution or ecological 

 importance. It is a morphological concept. Taxa classed as " forms " 

 are assumed to be unstable and to be subject to change if any alteration 

 occurs in the conditions which have produced them. It scarcely needs to be 

 pointed out that these are artificial groupings and that opinions about 

 individual cases may be modified by new evidence. The variety may prove 

 to be due to environment or, on the other hand, the form may prove to be a 

 stable variation. Two well-known examples are the following: Plantago 

 coronopiis L. var. pygmaea Lange, is a minute plant of dry places with entire, 



