1917.] 59 



only oiie centre, from which it made its way outwards to cover a range 

 ■wliose limits are determined by a hirge number of factors, such as the 

 age of the species, the existence of barriers to further progress, the 

 physiographical and climatic conditions, the presence of the necessary 

 food, the existence of enemies (whether predatory or parasitic), the 

 competition of allied and other forms with a similar habitat or pabu- 

 lum, etc. As a general rule, a species is most abundant in its centre 

 of origin, and becomes increasingly scarce as we recede from it. Our 

 own islands are almost too small, and too short a period has elapsed 

 since the Ice Age to permit this phenomenon to be definitely shown 

 in its full detail, but students of eveiy Order will be able readily to 

 quote examples of species which become rarer from north to south, 

 i.e., those of northern origin ; or from south-east to north and west,* i.e., 

 those of eastern origin ; or from south-east to north and east, i.e., those 

 of Lusitanian origin. Taking the CoJeoptera only, as examples of the 

 first we may quote Mii^codera arctica Payk., Agobiif' arcfims Payk., and 

 RhiKjium inquisitor L. (^indagator Brit. Cat.) ; of the second, many 

 species of Harpalus, Hygrobia (Pelohius) tarda Herbst, and Paederus 

 ripariu.s L. ; and of the third, Eurynebria complanata L., Phospthnga. 

 s?f?>ro//M?(7a/a Steph.,and Pentarthrnm hvftoni Woll. Tluisthe rarity of 

 a species in a given locality may be due to the fact that it is on the 

 confines of its range. 



2. — Extension of Range. 



Under certain favourable conditions such an organisation can 

 extend its range, gradually overspreading new tracts of country and 

 becoming common in places where it was once scarce or even absent. 

 Thus what was formerly a highly desirable acquisition for the cabinet 

 becomes in time almost a commonplace of collecting. This spread of 

 species, with the gradual change from rarity to frequency, is one of 

 the most interesting subjects in the study of Zoogeography, although 

 the determining factors of this extension of range are not always 

 obvious. In some cases man is certainly directly responsible, as in 

 the spread of the house mouse (Mns musculiis L.), and the brown rat 

 {Mils decnmanus Pall.) to all parts of the earth ; the occurrence nowa- 

 days of the snail,t Helix aspersa Miill., in many remote regions^ ; the 

 swarms of the Gripsy Moth {Ocneria dispar L.) in the New England 

 States, and the ubiquity of granary species of CoUemhola, G^'leo- 



' !Soe D. & R. F. .Scharff, " Europoaii Animals." Jjondoii, I'JOT ; 



Do. do. "The Hi.story of the Euroiieaii Fauna." London, 1899 ; 



See W. E. .Sharp, " Entomologi.sts' Record, ' 1901, etc. 

 + Scharff. "The History of tlie European Fauna." C. 1. 



X I have my.self observed Ildix iisprrsa completely naturaUscd and abundant in Cliile, 

 >'ew South WaleSj New Zealand, aij^l >'yvy Caledonia.— J.J. W, 



