92 THE VARIATION OF ANIMALS IN NATURE 



that they are often broken up into small self-contained com- 

 munities such as ' schools,' colonies, rookeries and shoals. 

 The statistical constitution of such communities is very little 

 known and only the colonies into which the populations of 

 land snails are divisible are at all well studied. Some 

 progress has been made with the study of the shoals of 

 commercial fishes (Schnakenbeck, 1931). The distinction 

 between such intimate subdivisions of a population and, 

 e.g., the races of ^joarces described by Schmidt, is not easy to 

 draw. 



A. Sporadic Individual Variation. — There seem to be 

 two main tendencies to be recognised under this head according 

 as a sporadic variation occurs throughout the range of a species 

 or is more restricted in its occurrence. The most obvious and 

 commonest type of individual variation of this kind is seen in 

 colour-phases of various sorts. We ought also to include 

 certain pattern-forms which occur rarely and sporadically, 

 e.g. in populations of land snails, in which the main pattern- 

 types show local statistical differences. 



The following are the principal ways in which individual 

 variants are distributed : 



(a) A typical form and a variation occur sporadically 

 throughout the range. 



1. Albinism. — The majority of species of mammals 

 which have been adequately investigated are found 

 occasionally to produce albinos in nature. Twenty- 

 one out of the forty-three British mammals dealt 

 with by Barrett-Hamilton and Hinton are known to 

 have produced albinos sporadically within the British 

 Isles. Similar sporadic variation is widespread in 

 birds and in some Lepidoptera. Though it is rare 

 in fishes, Norman (1931, p. 227) says that it is 

 common in flatfishes. 



2. The variety caeruleopunctata of the Small Copper Butterfly, 

 Heodes phlaeas. — Ford (1924) shows that this variety, 

 in which the upper side of the hind wings has marginal 

 blue spots, occurs sporadically through the greater 

 part of the range. It tends to occur in different 

 proportions in different places ; the ratio may remain 

 constant over a number of years. 



