238 Genetical Studies in Moths 



in Durham when compared with the conditions in Cleveland is due to 

 the prolonged industrialisation of the more northern district and the com- 

 paratively recent development of Teeside Blast Furnaces and Chemical 

 Works. 



We must guard against assuming that the lichens are entirely gone, 

 this is far from true, as careful search will indicate that many exist on 

 walls and trees in the immature granulose condition. Their passage to 

 the perfect cqndition has been prevented by the surrounding smoke 

 and more particularly by the hydrocarbons of the ethylene series — not 

 necessarily gaseous — it contains. Should anyone doubt the efficacy of 

 this effect a simple test will demonstrate its truth. Plant a few sweet 

 peas in a flower pot in the dark ; let them grow until the etiolated 

 epicotyl has attained some length. Then expose to town smoke. The 

 epicotyl is at once flattened and very serious disturbances in growth 

 ensue. 



I have not elaborated this point simply to show that smoke affects 

 vegetation but to show that such interference, whilst progressively dimin- 

 ishing as one leaves urban conditions, still possesses enormous powers at 

 great distances from the source of the contamination. Now if the smoke, 

 in addition to destroying vegetation, likewise conduces to melanism, 

 then when continuous melanism develops it should likewise diminish 

 as one leaves the town. Of the truth of this I have ample evidence 

 drawn from both districts and seen in Oporabia dilutata, one of the very 

 species we are studying. In Long Acre Dene Wood (1 ), probably the first 

 due south of Gateshead and Newcastle to yield the insect in quantity, a 

 good proportion are perfectly black, whilst those in Leybum Hold Dene (2) 

 are greenish black suffused forms, displaying the usual transverse bands ; 

 passing south of this to Lambton Woods (3) we find paler although still 

 melanochroic forms ; then finally we reach the Hermitage Wood (4) 

 where, in addition to the Lambton forms, we capture ordinary pale typical 

 insects. 



In the same valley Ypsipetes trifasciata^ shows the same progression 

 in its melanism as we approach Gateshead from Waldridge Fell (5). In 

 the latter locality the pale central band of the type still retains relics of 

 its former silveriness, whilst just beyond Lamesley in the habitat nearest 

 Gateshead all of the insects appertain to the completely black form 

 known as nigerrima. The case of Y. trifasciata is worthy of special 



1 The " T " on the general maps of the Team Valley and of the Northern Clevelands 

 shows the distribution of Ypsipetes trifasciata ; the " E" and " S " give similar facts for 

 Y, ruberata and Y. sordidata, respectively. 



