126 THE VARIATION OF ANIMALS IN NATURE 



are examined statistically (Sumner, Schmidt), the same 

 quantitative local divergences are discovered as those observed 

 in populations of land snails. It seems to be true on the 

 whole that there is a lack of innumerable individual variations 

 in vertebrates that requires explanation, though the obser- 

 vations of Fowler and Bean (1929) on variation in fishes of 

 the order Gapriformes must prepare us to realise that indi- 

 vidual variation is far more frequent than Rensch has allowed ; 

 but perhaps the wider range and consequent less susceptibility 

 to minor isolating influences render their populations more 

 homogeneous. It is also possible that a more highly evolved 

 physiological control makes them less susceptible to external 

 factors. A study of the variability of sedentary mammals 

 (such as small rodents) contrasted with that of more widely 

 ranging forms (carnivores and ruminants) is much to be 

 desired. 



2. The very frequent occurrence of variants established as 

 a small percentage of a population and at the same time living 

 along with the typical forms seems to us of some importance. 

 Many more examples are available of this phenomenon than 

 those which we have cited. 



3. The frequent occurrence of statistical divergences calls 

 for attention. It is not without significance that, when 

 populations are broken up by divergences of this kind (p. 99), 

 the latter can be maintained over periods of about ten years, 

 at least as far as the admittedly imperfect records allow us to 

 judge. As to the origin of these divergences it seems most 

 unlikely that they are due to selection. They sometimes occur 

 under identical ecological and bionomic conditions and, 

 unless we appeal to the argumentum ad ignorantiam, are most 

 unlikely to be produced by selective adaptation to local con- 

 ditions. For a similar reason they do not appear to be produced 

 by the direct effect of the environment. We are thus forced 

 to conclude that they are produced by the effects of local 

 isolation or obligatory preferential mating working on available 

 stocks of hereditary material. 



4. We have introduced somewhat cautiously the idea that 

 certain species have a more marked proneness to local and 

 regional variation than others, apart from any habits, etc., 

 which might promote this feature. The contrast between 

 the South American Wren and Buarremon (p. 106) is an 



