8 THE VARIATION OF ANIMALS IN NATURE 



obscure by the relatively slight amount of precise knowledge 

 as to which variants are heritable and which are mere fluctua- 

 tions. Every population will contain a certain element of 

 individual forms having the latter status and sometimes 

 (possibly quite often) large sections of a population will be of 

 this nature ; this is particularly true of plastic organisms, such 

 as corals and hydroids, in which ' ecological types,' the 

 products of the peculiar environmental conditions found in 

 various habitats, have been often reported. 



When fluctuations have been allowed for, as far as possible, 

 we are left with the important heritable elements. Of these 

 we may distinguish three kinds — (i) individual variants ; 

 (2) groups ; and (3) special categories of various types. 



(1) Individual Variants. — Individual variants occur in 

 nature with very different frequencies and there is every 

 gradation between the variant which occurs sporadically 

 throughout a population and groups of appreciable size. In 

 some classes and orders sporadic individual variation is common; 

 in others, group-formation is more characteristic. The diver- 

 gence of such individual variants may be in one or several 

 characters. 



(2) Groups . — Although no two individuals are ever exactly 

 alike in all their characters, it is a commonplace that indi- 

 viduals can be classed together in assemblages or groups of 

 various kinds. For the study of the origin of variation the 

 constitution and status of such groups are irrelevant, but, inas- 

 much as we find that variant individuals tend to form groups 

 characterised by the possession of a set of common and peculiar 

 characters and that such group-formation seems to be the 

 initial stage of evolutionary divergence, it is clearly part of 

 our business to inquire into the process by which recognisable 

 groups are formed. 



These groups differ among themselves not only in their 

 degree of distinctness, but also in the nature of the distinction. 

 Thus a clone is a different kind of assemblage from a physio- 

 logical race. The various kinds of groups recognised are 

 discussed in Chapter III. For the moment we are concerned 

 only with a single general question, viz. the relation between 

 taxonomic units and the concept of the natural ' population.' 



The facts of variation, and indeed all the phenomena with 

 which the biologist deals, are most often given in association 



