IX. 
humidity, Mr. Tutt tells us of the “ startling suddenness with which 
many species, during the wet season of 1888, assumed a darker 
colour.” Yet the humidity from the Gulf Stream, while assumed to pro¬ 
duce melanic forms elsewhere, failed to affect the hues in Lancashire and 
Yorkshire till the present time. It does not appear to me that he has 
sufficiently considered the difference between what I may call the 
permanent melanism of parts of Scotland, of Ireland, and the Isle of 
Man, with that melanism which has sprung up around our large 
towns and in our manufacturing districts during our own time. Not 
so much a difference in the results produced, as in the circumstances 
under which the}' are produced. Can we find one condition common 
to all ? Mr. Tutt says humidity, but it is abundantly evident that 
humidity is a condition which constantly exists without melanic 
forms. Lord Walsingham has suggested another to which I must 
now refer. 
In dealing with the “ Melanism of High Latitudes ” Lord Walsing¬ 
ham showed how great an advantage it is for an insect to be dark 
coloured, under the conditions which obtain in such localities. 
Where frequent clouds and mists obscure the sun, the insect that by 
its hue, is enabled to take advantage of every passing gleam, will 
certainly, in the struggle for existence, compete favourably with those 
whose hues are less heat-absorbing, will perpetuate its race where 
they would fail, and transmit, by the laws of heredity, those characters 
to its offspring which had enabled it to fulfil the purpose of its 
existence. Though this address was directed to the melanism of 
High Latitudes only, before concluding it, he referred also to the 
melanism of our manufacturing districts and of the neighbourhood of 
our large towns. I have already quoted part of the passage, previous 
to which he said ; “We should expect to find varieties tending to 
assume absorbent colouring not only in the Arctic and Alpine regions, 
where the amount of cloud and mist must greatly diminish the 
incidence of direct sunlight, but also under any conditions which 
would limit it in an artificial manner. Dr. J. Hann strongly insists 
upon the chemical energy and potentially of diffused sunlight ; 
corroborating and supplementing Tyndall’s observations to this effect.” 
Here, in my opinion, is the true solution. Whatever obstructs the 
direct rays of the sun, will increase the tendency to melanism, and 
though various conditions may be found, the action is the same and 
the result is the same. Whether these direct rays are obstructed by 
cloud and mist in the North and in mountainous regions by the 
sunless skies of Ireland and the Isle of Man, or by the smoky 
pail that hangs over our great cities, and our manufacturing 
districts, the effect is the same. The sun’s rays are obscured, and 
heat-absorbing colours are found on the wings of our Lepidoptera. 
