THE DISTRIBUTION OF VARIANTS IN NATURE 1 19 



a distinctive pattern ; possibly in some of them the mean of 

 the colony would not actually be repeated anywhere else in 

 the range of the species. But such colonies are so numerous, 

 and so often show a considerable range of variation, that it 

 is useless to name them all. Thus, while taxonomic procedure 

 has very good practical arguments in its favour, it tends to 

 exhibit geographical variation more distinct from other types 

 of variation than it really is. 



Physiological Races (see also Chapter III, p. 73). — There 

 is no theoretical reason to suppose that the physiological 

 (instinctive, psychical, etiological, etc.) characters of species 

 should be less variable than the morphological except in so 

 far as variation in the latter is less likely to impair viability. 

 In the Protozoa, strains differing in various physiological 

 properties (immunity and virulence) have long been known. 

 The literature of entomology, ornithology, etc., is full of 

 descriptions of individuals with aberrant habits or instincts. 

 In most cases, however, the previous history of the individual 

 was unknown, so that little can be concluded except that 

 instinct is capable of modification. It is easier to study the 

 phenomenon when a whole population exhibits such a change. 

 Such populations are termed ' biological races ' or ' physio- 

 logical strains ' of the species concerned. If physiological 

 characters are inherited in the same way as morphological, 

 the same tendency to group-formation and subdivision of the 

 species might be expected in them, some groups being charac- 

 terised mainly physiologically, others mainly morphologically. 

 A very much more complete knowledge of animals than we 

 possess might perhaps break down the distinction. 



Some of the data as to biological races are considered 

 elsewhere (Chapters II, III and VII), so that we shall en- 

 deavour here mainly to establish that physiological differen- 

 tiation occurs in all degrees. As an instance of the asso- 

 ciation of minute physiological differences associated with 

 almost equally small structural ones, we may mention the 

 work of Bodenheimer and Klein (1930), who deal with three 

 subspecies of the ant Messor semirufus in relation to temperature. 

 It was found that each race had a different optimum tempera- 

 ture for normal activities (viz. 18-4°, 19 , 20-3° C). This 

 and similar evidence that is now accumulating show that 

 at all grades of morphological differentiation physiological 



