i 5 8 THE BRITISH NATURALIST. [August 



His thesis then would stand thus : — 



Humidity, in the form of fog or rain, precipitates the soot suspended 

 in the air of Lancashire and Cheshire, staining all objects more or less 

 black, assimilative protection comes into play, melanism becomes at 

 once advantageous, and the thing is explained. We grant at once that 

 here is a tenable theory of the melanism of Lancashire and Cheshire 

 granting certain premises, but it is obviously impossible to explain the 

 melanism of Sligo and Shetland, by a theory which demands the 

 constant presence in the air of suspended soot. Still humidity must 

 be the agent, therefore the proposition is thus distorted. — Humidity 

 means clouds, clouds obscure the sun, black owing to greater absorbant 

 power is the best colour for the insect where the sunshine is scanty 

 and often obscured, hence derivative advantage again and the same 

 result. Did it never occur to Mr. Tutt that if humidity meant clouds, 

 and clouds melanism in Donegal and Caithness ever since those 

 localities had any insect population at all, then humidity and conse- 

 quent clouds fn Lancashire and Yorkshire should have been equally 

 operative since the beginning, without waiting for the adventitious aid 

 of the smoke and fumes of manufactures. 



However, ignoring such a difficulty as this, will it do to assume 

 roughly that great humidity and a strong melanic tendency are 

 generally coincident ? I think not. Of course the real issue is very 

 much complicated and obscured by the fact, that localities of extreme 

 humidity are generally localities of high elevations as well, and the 

 phenomena may easily be attributed to the wrong factor ; but dissoci- 

 ating as far as possible mere elevation from the question, any map show- 

 ing average rainfall should convince Mr. Tutt that Kerry, Carnarvon- 

 shire, and Cumberland are far ahead of Shetland or Orkney as regards 

 humidity, but which of these districts exhibits the most pronounced forms 

 of melanism ? I think it would be quite possible to argue that melanism 

 of what I have called the archaic sort, follows lines more of altitude 

 and latitude, than of humidity and longitude. This, of course, is prin- 

 cipally a matter of opinion and the detailed examination of evidence. 

 My object is rather to point out some of the instances of confused 

 thought and defective reasoning which occur in Mr. Tutt's book. 



His obvious fallacies also require notice. Firstly, I should not 

 have imagined that anyone would have attributed to the Gulf Stream 

 the humidity of our climate as Mr. Tutt has done (p. 40 et seq.). The 

 Gulf Stream does not influence the rain- fall of this country in the 

 slightest, the temperature it does very considerably, the excessive rain- 

 fall of our western coasts is due to the fact that they first intercept 

 and condense the prevailing south-westerly winds, laden as they are 

 with aqueous vapour, derived not specially from the Gulf Stream, but 

 from all the tropical Atlantic. This error does not, however, affect 



