THE CLASSIFICATION OF PLANTS 2123 



that the isolation may be of many kinds. It might also be added that the 

 permanency is not absolute. Definitions similar to the latter have been 

 given by several other writers, with a common emphasis on the genetic or 

 reproductive isolation of a group or population as the basic criterion for 

 specific discrimination. Plainly these two definitions are not referring 

 to the same thing. They present the two horns of what we have called the 

 Taxonomic Dilemma. 



If both definitions are legitimate, each from its own point of view, 

 then one is forced to recognize that there are really two sorts of species, 

 Museum Species and Natural Species. The museum species are to some 

 extent, at least, conventional. They are the logical species, they are adopted 

 and described upon the evidence of characters which can be readily seen 

 and preserved. They serve an essential purpose, to enable specimens to be 

 named and classified among other specimens to which they show the maxi- 

 mum of resemblance, but they are conceived as subjective entities and they 

 are not necessarily natural units. Museum species are well known and 

 understood and manv people think only of them when species are discussed. 



Natural species on the other hand are relatively little known and their 

 very existence may be considered disputable in some quarters. They are 

 conceived as being objective entities and they cannot be created by 

 systematists, they have to be discovered. To this end every source of 

 information must be tried; evidence from morphology, anatomy, bio- 

 chemistry, cytology, genetics and ecology must be sought, and even when 

 all this has been done it is possible that they may bear no recognizable 

 signature, no sure mark by which to know them, so that they remain 

 impalpable, hovering ghostlike behind the screen of the more tangible 

 museum species. Yet they are the ultimate taxonomic units, of which our 

 museum species are only reflections, and until we come to terms of under- 

 standing with them the keys to the problem of evolution will not be in our 



hands. 



Although they are so different in conception, the two categories are not 

 necessarily opposed. Museum species are generally based upon morpho- 

 logical characters but this is not so partial as it appears, for morphology is, 

 after all, the expression of inner constitution. Experience has shown in 

 certain cases that extended investigation by other means, e.g., cytology or 

 ecology, which was aimed at disentangling natural species from a museum 

 group, has only served to confirm the majority of the conclusions which 

 had previously been formed on a morphological basis. The criterion that 

 species are groups of individuals which breed only within their own limits 

 may therefore be satisfied in a number of cases by the museum species and 

 to this extent the dilemma is relieved. We may take it that the museum 

 method does frequently delimit the maximum natural species. It fails, 

 however, in delimiting the minimum species, for there are numerous cases 

 in which it is impossible to discriminate natural species by morphological 

 methods. This is most clearly true in some of the lowest groups, especially 

 the Bacteria, where species, or what are called species, rest almost entirely 



