236 Genetical Studies in Moths 



nigra. Owing to the evidence of S. lubricipeda and the other insects 

 named above, we are thus forced to accept the position that the exact 

 cause inducing the melanism acts or becomes potent for future action 

 during the larval life of the insect, and with certain insects very early in 

 that life. This incidentally explains why the so-called temperature 

 aberrations cannot reproduce their kind. 



The phenomenon being one initiated in larval life, it seems physically 

 impossible for environmental moisture to act through the soma so as to 

 affect at one and the same time the cells of the soma and those germinal 

 in character. We are therefore driven to search for other causes. 



I am quite aware that mechanical and physical disturbances, such as 

 centrifugal force and X-rays, have been shown capable of producing 

 somatic melanism, but each of these agencies acts at the so-called critical 

 point of early pupal life, and by crippling special cells at the one time, 

 and the whole organism at others. For these reasons it would be profit- 

 less to discuss them at length here. 



Such being the case what have I to offer in their place ? Let us con- 

 sider what other drastic changes have been noted in the affected areas 

 under consideration simultaneously with the appearance of melanism. 

 The most striking is the decadence of cryptogams, more particularly of 

 the mosses, liverworts and lichens, and to a less degree the ferns. This 

 most certainly originates from smoke contamination. No one who has 

 studied the cryptogamic botany of North Durham and North Yorks 

 would fail to contrast the rich moss and lichen floras recorded for Long 

 Acre Wood (1)^ in the Team Valley by Winch ninety years ago with their 

 paucity now. Of the lichens only the ubiquitous and apparently iron- 

 constitutioned Cladonia pyxidata persists, and of the mosses only the 

 accommodating Fissidentes, Mnium hornum, Hypna and a few others 

 remain. The same holds good but to a less extent in Cleveland ; there 

 one would look in vain in the northern portions for the hosts of lichens 

 listed by Mudd half a century ago. 



But in the two cases differences are easily recognised ; in the Durham 

 locality, whilst matters with the mosses slightly ameliorate as we leave 

 the soot zone, the apparent extermination of the lichens has been more 

 complete. Besides Cladonia pyxidata I am only acquainted with two 

 patches of well-grown lichens in the Team Valley, and both of these are 

 Parmelia saxatilis. In the Clevelands as one leaves Middlesbrough the 

 lichens reappear, pari passu, so that in the remoter dales the whole flora 

 comprises the same species and in the same abundance as fifty years ago. 

 1 The figures refer to those on the map shown on Fig. 11. 



