io 4 THE VARIATION OF ANIMALS IN NATURE 



Mexico, one found in Florida, Mexico, Cuba and Venezuela, 

 and a third found in Florida and the Greater Antilles. In 

 C. flavitarsus there are four forms, which overlap in a rather 

 similar way, but there is a main type in U.S.A. and another 

 in S. America. 



C. Geographical Variation. — Under this heading we 

 propose to deal with instances illustrating the tendency seen 

 in the species of certain groups to be divisible into subordinate 

 groups occupying separate or overlapping areas. Such groups 

 are usually alluded to as subspecies (p. 63). 



We have already had occasion to contrast the frequent 

 occurrence of this kind of geographical variation in Vertebrates 

 with the irregular and more complex distributional phenomena 

 in the Invertebrates. This point required some further dis- 

 cussion. So far as we are aware Rensch (1929) was the first 

 to point out and to stress the fact that species of certain groups 

 are more obviously divisible into geographical races than 

 those of others. Admitting the inadequacy of taxonomic 

 study and the slight amount of attention paid so far to the 

 study of geographical variation in some groups, he considers 

 that mammals, birds, reptiles and Amphibia, Coleoptera, 

 Lepidoptera, Hymenoptera and Orthoptera display this 

 tendency markedly. The other insect groups, Arachnids 

 and Myriopods, probably show the same tendency, but the 

 available knowledge is defective. The tendency is seen in 

 land molluscs, but is largely masked by individual and ' ecolo- 

 gical ' variation. Freshwater and marine groups show it 

 in some measure ; but it is less marked here. Rensch's actual 

 survey of the chief groups of the animal kingdom is not 

 exhaustive, but it includes the more important groups. 



He explains (p. 79) the difference in the incidence of 

 geographical variation by pointing out that in certain groups 

 the habits, size and mode of reproduction are of such a nature 

 as to prevent the establishment of barriers and so of isolation 

 between the parts of a population. Migratory habits, as in 

 many seabirds and fishes, small size which facilitates accidental 

 transport, as in land snails, Tardigrada, Nematoda, etc., and 

 the occurrence of ' resting eggs ' as in Cladocera (but cf. 

 Lowndes, 1930 ; see on p. 135) are all factors which make 

 for the homogeneity of a population. 



Rensch contrasts the uniformity of the widely ranging 



