48 THE VARIATION OF ANIMALS IN NATURE 



legs) may be considered as adaptations of different biological 

 ends. It is probable that these characters have been developed 

 by means of natural selection. Other characteristics like the 

 general size of the body and coloration cannot at the present 

 moment be even hypothetically evaluated as having any 

 biological importance for the organism.' 



Allen's Law is corroborated in the diminished length of 

 the tails of the island races of certain British mice. It is 

 possible, however, that there is a further special effect due to 

 island life. In all the British mice and shrews which have 

 races in the Shetlands, Orkneys and Hebrides, the proportion 

 of the tail-length to that of the head and body is almost 

 invariably less in the island races (figures taken from Barrett- 

 Hamilton and Hinton, '1910-21). Unfortunately, the majority 

 of the insular forms are found in the north, while the measure- 

 ments of the mainland forms were based on southern speci- 

 mens, so that it is not possible to separate the effects of latitude 

 from those of insular life. 



Le Souef (1930) has published an interesting note on the 

 changes of three species of wallaby and an opossum imported 

 sixty years ago from Australia to New Zealand. All have 

 varied in the same way, the fur being now longer, more silky, 

 and less dense. 



Rensch's ornithological examples illustrating Gloger's Law 

 may be supplemented by the data brought forward by Banks 

 (1925). Here a number of subspecies or of specimens from 

 different parts of the range of a species were compared and 

 their colours correlated with the average meteorological con- 

 ditions obtained during the breeding season. A very general 

 positive correlation was found to exist between temperature and 

 dark colours. The relation between pigmentation and humidity 

 is not nearly so simple, being sometimes inverse, sometimes 

 direct, but it appears in any case that the darkening effect of 

 higher temperature is evident only in the presence of a moderate 

 humidity. In areas which are very dry the colours tend to 

 be pale in spite of a high temperature. This general result 

 agrees with the well-known results which Beebe (1907) found 

 by experiment. Dealing with doves of the genus Scardafella 

 he found first that, in nature, there was a regular increase in 

 dark pigment as one passes from Mexico to Brazil, the centre 

 of least pigmentation being the driest area and the pigment 



