240 Geiietical Studies in Moths 



attention because twenty-six years ago the insect was quite typical 

 locally. 



Near Middlesbrough 0. dilutata is melanic right up to the base of the 

 hills seven miles away ; over the first ridge and not a mile away from 

 the limit of melanism the moth is as pale as if taken in Ireland. In 

 this district Ypsipetes trifasciata is more local ; still there are two 

 stations for it, one on the south face of Eston Moor and the other in 

 Lonsdale, farther south. The insects from Eston are melanochroic whilst 

 the others are quite ordinary. 



But other species show the gradual decrease in melanism much more 

 vividly as we recede from town conditions in Northern Cleveland. 

 Boarmia repandata near the town is quite black ; just three miles out 

 it is grey, whilst specimens from Stokesley (see Fig.' 13) are perfectly 

 typical. So, too, near Middlesbrough and North Ormesby Hyhernia 

 marginaria exhibits total melanism ; beyond a belt of three miles such 

 a form is unknown. 



Of the two areas the Middlesbrough one is the more striking ; as we 

 leave the borough the annual rainfall increases rapidly from 24*9 inches 

 to 36 inches — reaching its maximum where the melanism is least. 

 Instead of a direct relationship between rainfall and melanism an 

 inverse one is manifested. 



As a deduction from the facts laid down above I have satisfied 

 myself on two points : (1) as far as urban areas are concerned moisture 

 plays, if any, a very minor part ; (2) the only agents correlated with the 

 melanism in its incidence are the smoke and impurities thrown into the 

 atmosphere through the springing up of great manufacturing towns. 

 How the latter factor may conceivably act detailed consideration will 

 determine ; melanism in other than urban districts we shall reserve for 

 treatment later. 



Let us pause here to examine the chemical status of the black 

 pigments found in the animal world generally, in order to equip ourselves 

 for our proposed investigation as to their origin in the lepidoptera. 

 These pigments known as melanins are not necessarily black ; indeed, 

 some are brown. They occur in skin, hair, wool, feathers, scales, muscles 

 and various other parts of the bodies of animals of diverse affinities. 

 In composition they are substances closely allied to proteins. In fact, 

 as far as it is humanly possible to decide they are chemically the same 

 as the humins, a series of bodies derived artificially from many albuminous 

 substances by the action of enzymes or ferments. 



Of the albumin-derived proteins tyrosin (parahydroxyphenylamino- 



