J. W. H. Harrison 213 



ditions approached the normal, by gradual infiltration inland until all 

 the heather clad hills of later glacial times were held. Then a slowing- 

 up would intervene, for invading northern forms which had utilised the 

 Scandinavian route as an inter-glacial means of escape from south-eastern 

 Europe would act as a check until recurring glacial conditions restricted 

 the movements of both groups. 



Finally the ice vanished and ground capable of colonisation was once 

 more open and to some extent regained, but once again slowly, for we 

 must not forget that Galluna is a plant of south-western origin and was 

 passing from more into less favourable areas, and this tardiness kept back 

 the insect, with the result that before the continent was attained the 

 British Islands had come into being. 



Advance south-eastward, likewise, was slow ; in that direction dense 

 masses of migrating southern forms blocked the way until at length 

 when equilibrium was set up Great Britain was separated from Ireland 

 and the western islands from both ; thus the insect was in possession of 

 habitats closely approximating to those it holds today ; if one thing is 

 more certain than any other concerning British vegetation it is that the 

 bulk of our heather clad moorlands with their peculiar plant for- 

 mations are more primitive, i.e. are in a more natural state, than any 

 others. 



Accustomed as 0. filigrammaria had become to a diet of ericaceous 

 plants during the ice age, it had not lost the faculty of feeding up easily 

 and satisfactorily on the Betula, Alrms and other plants of its earlier days. 

 Yet despite this, even in the birch-heather associations of moorland 

 slopes, the female instinctively lays its eggs on Galluna or Erica, the 

 birch being entirely neglected. Year after year I have beaten birches in 

 such associations — even seedling trees not so tall as the surrounding 

 heather — on a moor where both type 0. autumnata and the subspecies 

 . filigrammaria occur, and never has the birch i^rodncedi filigrammaria 

 nor the heather autumnata. And this is the more wonderful when one 

 considers the ease with which the confined insect oviposits in chip boxes, 

 on muslin threads, in cotton wool and the like, and on trees when in the 

 semicaptivity of a roomy muslin cage. How has this instinctive choice 

 of Galluna and Erica become engrained in the germ plasm ? It seems 

 very unlikely that it is mutational, and impossible that it should be 

 selectional when due regard is paid to the facts outlined above; once 

 again the evidence strongly urges one to the Lamarckian view that long 

 years of compulsory oviposition on these plants, whether because they 

 were ericaceous or simply because they were low-growing or because of 



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