228 Geiietical Studies in Moths 



Only one further fact remains to be brought forward, and we are 

 finished with the local races ; in spite of the presence of many larches 

 never once have I beaten a larva from that tree in the alderwood, yet 

 every single individual of the few birches, whether in the open spaces 

 or amongst the alders, has yielded the larva, which conclusively proves 

 that the larch diet in the pinewood was only assumed as a last resort. 



{d) The tase of Oporabia dilutata and its subspecies O. christyi. 



To describe in detail the pre-glacial migrations of these two forms 

 here would be unprofitable ; firstly those of 0. dilutata agree too closely 

 with those of Apocheima hispidaria, which I have worked out previously ^, 

 and secondly the recognition of christyi as a subspecies has been too 

 recent to admit of its present distribution being worked out, and thence 

 its movements in the past deduced. We shall therefore confine our- 

 selves to a brief study of the Cleveland local races of 0. dilutata. 



Laying aside the case of 0. christyi as being impossible of study for 

 reasons similar to those just adduced, its local habitats not being fully 

 ascertained, we are left with two recognisable races. One of these 

 is the ordinary sufiused melanic form with a considerable range of 

 fluctuating variation and the other a brilliant silvery form attached to 

 the little island of oaks within the coniferous and alderwood in Lonsdale. 

 Although separated by a ridge 1000 feet high, the nearest colony of 

 0. 'dilutata to the latter is barely three-quarters of a mile away in 

 Easby oakwood, where the species exhibits melanism of a most pro- 

 nounced type. 



As regards the oak island two views are possible : first that it 

 represents the remains of Sir Charles Turner's unfortunate experiments 

 in 1782; and second that, if the presence of Scilla nutans and Holcus 

 mollis can be used as corroborative evidence, it represents the surviving 

 portions of primaeval oak woods, long since demolished otherwise by 

 human interference, which have been subsequently isolated by the 

 planting of alder, larch and fir. 



In any case it must have been cut off for over a hundred years 

 from the oaks in Easby and Airyholme Woods, with which of old it 

 • may have been continuous around the spur of the hill between Lonsdale 

 and Kildale. Segregated from influences of the nature of directly in- 

 duced and of infiltrated melanism so disturbing in these other colonies 



^ Harrison, '' The Geographical Distribution of the Moths of the Geometrid Subfamily 

 Bistoninae," Naturalist, pp. 317—320 (1917). 



