INTRODUCTION n 



but we are concerned with the character of the population 

 rather than with the name given to it. In the study of such 

 populations we can use for convenience any name that may 

 have been given by a taxonomist, even though groups put into 

 the same taxonomic category are not necessarily equivalent 

 in degree of isolation or divergence. Nevertheless the dis- 

 tribution of variants in nature does not, in general, appear 

 to be at random ; they are arranged so that different types of 

 populations can be recognised. Populations may be distin- 

 guished by a varying number of physiological and structural 

 characters which may be correlated in different degrees with 

 one another. Further, the size of the area inhabited and the 

 nature of the factors limiting the area may differ. 



Topographical groups. — By far the most striking manifestation 

 of natural variation is the occurrence within a population of 

 larger or smaller groups of some measure of homogeneity. 

 Usually these are denned by at least partial isolation and they 

 range in size from a small patch of individuals (colony), pecu- 

 liarly characteristic of small sedentary animals like land snails, 

 to a group occupying an extensive area (geographical race). 

 Such groups may be rigorously isolated from neighbouring 

 races, or they may overlap. In the growth of these assemblages 

 we may note as in (i) that the divergence of one or more 

 characters may be involved. 



(3) Special Categories. — The terms polymorphism and 

 dimorphism are sometimes used without any general agree- 

 ment as to their meaning and it is necessary to clear up this 

 ambiguity. In its clearest and most usually employed sense 

 dimorphism is applied to the occurrence within a species of 

 two strongly marked and discontinuous phases, such as we 

 see in the difference between the colour, etc., of males and 

 females, between seasonal forms, or between mimetic forms 

 {e.g. the East and West African female of Acraea alciope (Lepi- 

 doptera), Eltringham, 1910, pp. 44-45). The term has also 

 been given to other contrasted types within a species, whose 

 occurrence is not apparently related to bionomic needs, e.g. 

 by Bouvier (1904 : dimorphism of the Atyidae). 



Polymorphism has been used either in a general way to 

 denote that a population is very variable (cf. Coutagne, 1895) 

 or with a special significance to denote the occurrence of 

 several well-marked phases which inhabit the same area. 



