72 THE VARIATION OF ANIMALS IN NATURE 



exotype which depend on the recognition that a given form is 

 non-heritable. Perhaps we might also include the ecotype 

 and ecospecies (Turesson, Alpatov), which are combinations 

 of genetical and ecological concepts. Even in motile animals 

 such as ants Alpatov (1924) has been able to recognise 

 analogous ' subspecies ecologicae truncicolae ' in the European 

 and Japanese subspecies of Formica rufa. We are dealing 

 here, however, with a category having primarily an ecological 

 basis, some members of which are physiologically differentiated 

 (cf. Chapter IV, p. 119). 



In categories such as the clone and the pure line one may 

 say that the logical classificatory ideal of a category having 

 standardised characterisation is attained. These units are 

 defined not by their degree of morphological divergence, but 

 by their mode of reproduction and degree of genetical homo- 

 geneity. 



Some of the genetical units are obviously subdivisions of 

 the species. It has long been realised that taxonomic units 

 may contain numerous intercrossing strains (? = petites 

 especes),just as, considered in the time-relationship, the lineage 

 consists of interwoven and anastomosing lines of descent which 

 at any one horizon seem to have a similar status. Other such 

 categories have less to do with the content of the species. The 

 pure line is indeed an expression of differentiation within the 

 species, but as it is (sensu stricto) the result of a particular mode 

 of reproduction (autogamous), it is only of importance in 

 certain groups. It must also be noticed that a pure line may 

 consist of individuals homozygous for only one pair of allelo- 

 morphs. The term pure line is sometimes inaccurately given 

 to a genotypically homogeneous group, without reference to 

 the mode of reproduction, e.g. a homozygous biotype. Clone- 

 formation, on the other hand, seen in the Protozoa will be 

 characteristic only of such parts of a species-population as are 

 reproducing asexually. 1 



The term biotype (' a population consisting of individuals 

 with identical genotypical constitution ' (du Rietz) ) is a 

 recognisable entity among both autogamous and allogamous 

 forms, but, as du Rietz (I.e. p. 340) points out, there is little 

 chance that in regularly allogamous forms any biotype will 



1 The term clone is sometimes applied to the broods of parthenogenetic 

 animals. 



