60 [March, 



ptera, and Lepidoptera. In other cases, pressure of hunger in over- 

 popuhited (by the species) districts drives an animal out to seek fresh 

 food supplies, as in the case of locusts, and perhaps the Painted Lady 

 (Vanessa carclui L.) and other butterflies. Sometimes it seems to be 

 due to an unexplained mio-ratory instinct in the creature itself, possibly 

 of somewhat recent acquisition as, perhaps, in the case of Danaida 

 plexippns L.,* or possibly more deeply seated — an example of so-called 

 "race-memory" — as with the Lemming (Myodes lemmvs). Some- 

 times it is difficult to find any clear reason for the extension of the 

 range, e.g., Lathridius (Coninomus) nodifer Westw., and Plusia moneta 

 F., although one may make a number of likely suggestions, such as those 

 connected with food supply, climate, absence of competition, etc. It 

 is clear, however, that in cases of this kind the casual factor can be 

 recognised from the fact that the originally rare organism becomes 

 increasingly common, and tends to spread further afield. 



In some cases this extension of range is retained permanently, as 

 in the cases of the brown rat and the common cockroach (Stilopyga 

 orientalis L.). In other cases the species is gradually driven back 

 again by some inimical cause, and is to be found no more in its new 

 territory. For example, the beetle Hygrobia tarda Hei'bst seems 

 occasionally to come north, only to be driven back again, probably by 

 our northern weather conditions. Thus one specimen has been found 

 in the Newcastle district,! and in 1888 it occurred in numbers near 

 WithernseaJ in East Yorkshire, although it is now extinct there. 



In a few cases we can see the ebb and flow of the species, as in 

 the case of the cotton boll ■weevil§ (Anthonomus grandis), which in 

 favourable years spreads to the more northern American cotton 

 States, only to be killed off and become extinct in bad seasons, being 

 replaced by fresh immigrants in suitable years again. 



3. GrRADUAL RESTRICTION OF RaNGE. 



As opposed to the above, some insects ai-e losing in the struggle 

 for existence, either owing to competition with introduced or with 

 other local forms, or to the change in the physical environment brought 

 about in some cases by natural means, or more usually by man. 



* J. J. Walker, "Geographical Distribution of Vanaida ple.vippvs L." Eiit. Mo. Mag., 188.5-0, 

 pp. 217-224 ; 1914, 181-193, 224-237. 



t J. T. Bold, " Catalogue of the Coleoptrrn of Northumberland and Durhaui. 1871, p- l^J- 

 J York.s. Nat. Union Excursion Circular. No. 224. 

 § C. A. Ealand, " Insects and Man." London. 1915. 



