J. W. H. Harrison 243 



the food is unusual in districts wrapped in a constant mantle of chemical 

 smoke. Despite the truth of my observations that tree trunks are not 

 significantly darkened every leaf (I speak from my knowledge of the 

 Middlesbrough district) bears a film of smoke- derived impurities com- 

 posed of organic compounds, both aliphatic and aromatic, and various 

 salts of potassium, manganese, iron, sodium and other metals. If this 

 altered metabolism is as I postulated fitted to deal with quantities of 

 these ingested with the food, then without doubt it will endow its 

 possessor with a considerable start in the race of life. That foliage 

 thus contaminated is injurious to lepidopterous larvae may be readily 

 demonstrated. If larvae of such a species as Asphalia flavicornis, 

 originating in some remote Scotch wood, are reared on birch grown in 

 Middlesbrough they die one by one until all are gone. But it is quite 

 probable that such a radically changed metabolism as that laid down in 

 our theory above would render insects endowed with it more capable of 

 dealing with smoke-borne impurities ; so that those not possessing it 

 would commence life under a severe handicap when submitted to urban 

 conditions, and thus their ultimate supplanting by their afiected relatives 

 secured. Under this hypothesis the smoke itself would not directly 

 induce the melanism but would in the end secure its being supreme. 



There remains still another possibility; just as in Firth's^ and 

 MacDougal's 2 experiments the injection of various salts into the ovaries 

 of such plants as Epilobium roseum and Oenothera odorata is stated to have 

 produced noteworthy aberrational forms, so the smoke-derived chemical 

 compounds, organic and inorganic alike, may have affected the germ 

 cells of the insect. This action, whilst presumably most potent in early 

 larval life when relations between somatic and germ cells are most 

 intimate, could work at any larval stage ; by its action the chromosomes, 

 whether by detachment, replacement or removal of the side chains of 

 their essential nitrogenous compounds, could be so altered in powers as 

 to cause the setting up of a type of metabolism quite foreign to the 

 insect; this could be the type just discussed. Such a course need not 

 be that pursued by all of the insects under town influence any more 

 than all of Firth's and MacDougal's plants bore seeds yielding mutations; 

 such insects as did follow it, whether selection worked on colour, on 

 constitutional hardiness, or on better equipment for dealing with foreign 



1 Firth, " An elementary Inquiry as to the Origin of Species," Journal Army Med 

 Corps, Vol. XVI. pp. 497—504 (1912). 



- MacDougal, "Alterations in Heredity induced by Ovarial Treatments," Bot. Gazette, 

 Vol. LI. pp. 241—257 (1911). 



Journ. of Gen. ix 16 



