J. W. H. Harrison 221 



to this Wilton Wood, from a coniferous standpoint, is quite untouched, 

 whilst birch and alder alike are vanishing. 



From this we glean that the once continuous Eston colony of Oporabia 

 autumnata was, about the year 1800, broken into two distinct portions 

 separated by half a mile of heather and bracken ; from that period until 

 1860 and onward until 1885 these two sections, although disjoined, con- 

 tinued to exist under identical environment. After 1885 the conditions 

 fundamentally altered ; one division had to live in a coniferous wood in 

 which birch and alder were being suppressed, and the other in a birch- 

 wood (with some alder) where conifers were disappearing. 



With these changes corresponding changes of an evolutionary nature 

 have occurred in the respective portions of the old autumnata colony 

 which exhibit themselves especially in the following four features : 

 (1) the mean sizes of the two races are different ; (2) their food plants are 

 distinct — birch in the ofie case and larch (rarely pine) in the other; 

 (3) the Wilton Wood insect is a duller, feebly marked insect with its 

 variation range swinging round this as a modal condition, whereas the 

 birch insect presents us with diverse strains with an abundance of pale 

 silvery and barred forms as well as of feebly marked forms including 

 those just referred to; (4) the pine wood insect emerges in September 

 and is quite over by the end of that month ; on the contrary the birch 

 insect rarely occurs in September, in which month I have seen but two 

 wild specimens — a male in 1910 and a female in 1916 ; it only becomes 

 abundant about October 10. So great was the divergence in this re- 

 spect that for several years after my discovery of the insect amongst the 

 larclies, in spite of careful search conducted on the same days as I 

 climbed the hill specially to secure material from the pinewood, I missed 

 the birch colony and actually published a note to the effect that the 

 insect did not occur there. Only a casual visit in mid October, 1909, in 

 search of Arachnida, revealed its presence and demonstrated why it had 

 escaped previous observation. 



And the wonderful thing about these differences is that they must 

 have arisen in the period between 1886 (when the environments ceased to 

 be alike) and 1907, when my acquaintance with the insect commenced. 



That they do not depend upon the ground vegetation seems obvious 

 because amongst the larches, where alone the insect occurs in Wilton 

 Wood, the plant associations differ in no important respect from those 

 of the birch wood, as the following table, including for facility in refer- 

 ence the vegetation of the alderwood in Lonsdale, will show. 



The ground vegetation thus not being the decisive factor, it seems 



