212 Genetical Studies in Moths 



Another explanation of the earlier appearance is, however, possible. 

 This Galluna-ieeAmg autumnata which we will hereafter call fili- 

 grammaria differs in several minor structural points — cristae, octavals, 

 intensity of wing markings — as well as one major point, the loss of a 

 single chromosome. Now this latter divergence must be mutational in 

 origin ; one cannot conceive such a fundamental difference arising by 

 selection. It is therefore possible enough that the change in the life 

 history is intimately bound up with these differences in size and other 

 points, and that all arise from the mutation ini^olving the loss of a 

 chromosome, each tending to the conservation of the race and therefore 

 in the end characteristic of it. 



The same might be suggested as an explanation of the curiously 

 significant colour scheme of the larva, both in respect to the actual 

 colours and to its pattern. To this view, however, one great objection 

 can be advanced ; they approach too unmistakably to those obtaining 

 in the larvae of other heather feeders like Anarta myrtilli, Agrotis 

 agathina, Eupithecia nanata, and the case is too closely parallel to that 

 of all Pin us feeders, whether lepidopterous or hymenopterous, for the 

 matter to be one of a chance mutation. As is generally recognised, 

 mutations take place in all directions from the modal condition and 

 their precise course is a matter of chance ; therefore to explain the 

 convergence of the larval designs and colours in all the above ericetal 

 lepidoptera, appertaining to widely separated groups, on such a basis 

 seems too far fetched. We seem rather to be dealing with characters 

 evolved in direct response to environmental forces, like those in Poulton's 

 A niphidasys betalaria, and fixed by their long continued incidence, aided 

 perhaps by natural selection. 



To sum up, attaching due weight to all of the observed facts, chro- 

 mosome difference included, and considering their origin dispassionately ^ 

 in my opinion the divergences from type autumnata are best explained 

 as being brought about by stimuli of the environment, and thus more 

 readily explicable on Lamarckian grounds. 



Favourable inter-glacial interludes then saw the subspecies in its 

 present-day guise ready to seize any opportunity of advance, and the 

 waning of the ice, with the exposure of its huge deposits of glacial drift, 

 gave it unique scope for doing so, so readily does Galluna invade such 

 ground. But advance was bound to be northward ; thus the incipient 

 moorlands along the then common western shores of Scotland and Ireland 

 would be first colonised, followed, as the ice yielded still further and con- 

 ^ I commenced these studies from a pronounced anti-Lamarckian standpoint. 



