CLASSIFICATION 367 



are agreed in regarding all the Trout found in the British Isles 

 as belonging to a single very variable species. In the same way, 

 most anthropologists regard all living races of mankind as 

 representing one species of Homo, to which the name //omo 

 sapiens is given, but supposing that all the races were to die out, 

 with the exception of the European and the Bushman, these 

 two types would undoubtedly be placed in distinct species. 



A species, therefore, is not, and never can be, a fixed, 

 immutable unit, and no systematist living is able to lay down 

 any rule as to the amount of difference required to recognise a 

 species. He must inevitably be guided by conveniences and 

 by the circumstances of the particular case that he happens 

 to be studying. As Dr. Regan puts it: "In practice it often 

 happens that geographical forms, representing each other in 

 different areas, are given only sub-specific rank, even when they 

 are well defined, and that closely related forms,^ not easily 

 distinguished, are given specific rank when they inhabit the 

 same area but keep apart." Moreover, the value of a particular 

 morphological character, whether it be the form of the fins, 

 the structure of the scales, or the colour pattern, differs 

 enormously in dififerent famihes or orders, and a character 

 which serves to distinguish two species of, say, Cyprinids, niay 

 be of sufficient importance to separate two genera of Cichlids. 

 In spite of the huge collections preserved in some of the great 

 national museums of the world, many of the species known to 

 science have been described on the basis of one or two specimens, 

 som^etimes young, sometimes poorly preserved, and it is not 

 until every species is represented by a complete series of examples 

 illustrating its geographical range, variation, growth, seasonal 

 changes, sexual differences, and so on (a highly Utopian and 

 improbable state of affairs), that any sort of finality is to be 

 expected. As it is, almost every new species discovered modifies 

 in some way our conception of the relationship of the species 

 already known, and it not infrequently happens that the 

 discovery of one or two new forms leads to the coniplete 

 reclassification of the genus, or even of the family to which it 

 belongs. 



Yet another unit of classification, the sub-species, may come 

 between the individual and the species, and represents a com- 

 munity or group of related communities, whose distinguishing 

 features are not of sufficient importance to entitle them to 

 rank as a true species, but which, nevertheless, enable an expert 

 to separate them from other nearly related communities. The 



