322 THE VARIATION OF ANIMALS IN NATURE 



that the requisite mutations always turned up with each new 

 change of habitat. 



It may be questioned, indeed, whether in fact there are in 

 nature ' untenanted habitats ' available for the spread of a 

 species overflowing from its natural habitat. Some informa- 

 tion may be gained from the records of the rapid spread of 

 introduced species. 



(i) The Grey Squirrel (Sciurus carolinensis) , first introduced 

 into England about 1876, has now spread over a large part of 

 the south, west, and north-east counties. Middleton (1931, 

 pp. 79-80) has shown that this squirrel has ' stepped into a 

 practically vacant place in the British animal community,' 

 because the Red Squirrel, originally a pine-forest denizen, had 

 never over-populated the deciduous trees and that ' niche ' 

 was largely a vacant one. Moreover, the Red Squirrel, owing 

 to epidemics, was numerically at a low ebb. It is thus apparent 

 that there was in fact an untenanted habitat waiting for the 

 Grey Squirrel. 



(2) The Slipper Limpet {Crepidula fornicata) was first intro- 

 duced into England in 1886 (Robson, 1929) and has since then 

 spread round the east and south coasts, reaching as far west 

 as Swanage, in Dorset. It has principally occupied oyster- 

 beds, but may be found sporadically in other habitats. 



(3) The small Gastropod Paludestrinajenkinsi (Robson, 1923) 

 has similarly spread with great rapidity through the brackish 

 and fresh waters of Great Britain. 



(4) Similar cases are seen in Cordylophora lacustris (Harmer, 

 1 901) and Planorbis indicus (Robson, MS.). Thompson (1928, 

 p. 107), in discussing the spread of certain agricultural pests 

 (e.g. the Gypsy Moth, the European Cornborer), though he 

 allows that ' diminution of the intensity of causes of mortality 

 of the non-parasitic order may at times be responsible for the 

 increase and spread of introduced pests,' holds that ' the 

 absence of parasitic or predacious enemies is the real [more 

 frequent] cause of the increase of the imported species.' This 

 may be true enough ; but it must be added that, even if the 

 absence of predacious or parasitic enemies be a determining 

 factor, there must also be available enough food, shelter, etc., 

 to sustain the very noticeable natural increase. 



How far these examples are representative of the general 

 state of affairs in nature and whether we are entitled to assume 



