THE CATEGORIES OF VARIANT INDIVIDUALS 73 



be represented by more than one individual on account of the 

 great number of possible gene-combinations. 



Just as Rensch attempted {I.e.) to reconcile the systematic 

 and geographical concepts by a new terminology, so Lotsy 

 has attempted to synthesise systematic and genetical results. 

 He pointed out that the homozygous biotype is the only real 

 fundamental taxonomic unit (191 6) and therefore the only 

 unit worthy of being called species. He proposed the term 

 Jordanon to denominate the smaller character-groupings that 

 Jordan had detected within many Linnean species, and Linneon 

 for the larger composite groups. A considerable literature 

 has accumulated around Lotsy's suggestion. We do not 

 venture to discuss what is primarily a feature of plant popu- 

 lations. But there seems to be this much of common ground 

 between botanical and zoological results. As we have seen 

 in discussing Rensch's proposal, there are homogeneous and 

 heterogeneous species (' simple ' and ' compound,' Cockayne 

 and Allan, 1927) and the lines between a group consisting of 

 a single biotype and a Jordanon and between the latter and 

 a Linneon are quite arbitrary. What we seem to be dealing 

 with is the progressive formation of groups differing in more 

 and more characters. 



Genetical analysis has revealed a process of differentiation 

 partly produced by the mechanism of heredity, partly the 

 result of some other factor or factors. At the lowest level, 

 populations have their characteristics determined by the 

 processes of heredity and methods of reproduction — they are 

 homozygous or heterozygous, pure lines or heterogeneous 

 assemblages. Some characters may keep together in pairs 

 according to the amount of linkage. Imposed on this funda- 

 mental character-distribution is the process usually recognised 

 by the taxonomists by which larger and more substantial 

 character-groups are formed, either with or without geo- 

 graphical or ecological differentiation. 



V. Physiological Categories 



Of recent years it has been increasingly apparent that in 

 certain classes taxonomic species are subdivided into races, 

 characterised by slight or no morphological differences, but 

 by marked differences of habitat, food-preference and even of 



