THE BRITISH NATURALIST. 



69 



such forms or species might increase proportionately to others in wet 

 seasons. He says (p. 64) : " We want really to know whether 

 exposure to wet (or cold) makes dark varieties or only selects them. 

 I do not know that evidence of it making them is yet very cogent." 



I have now gone through those of the various suggestion that have 

 been made in this subject during the last 50 years, which appeared to 

 me to have an important bearing on the " Moisture producing 

 Melanism " theory, my extracts and comments being of course limited 

 by the time at my disposal. Though we are only at the beginning of 

 the enquiry, we know enough now to show the direction in which we 

 must search for the solution. Moisture does not afford it. Our 

 swamps and fens not only give us no melanic forms, but the species 

 that appear to be best fitted for these most humid localities are pale 

 ochreous or light drab in colour. Even Mr. Tutt can only suggest a 

 chance specimen or two, and these, though darker near smoky London, 

 are not so in similar places in Kent. Now it is quite certain if it were 

 moisture which made Nigricans black in Greenwich marshes, it ought 

 to produce the same effect in Kent, and the insect ought to be light 

 coloured on the dry sand-banks at Hartlepool. If the action of the 

 Gulf Stream had the effect Mr. Tutt supposes, it might explain the 

 permanent melanism of Ireland, the Isle of Man, and the North West 

 of Scotland, but it cannot be assumed to explain that of Lancashire, 

 Yorkshire, Durham, and various other similar places, it cannot explain 

 the melanism of London, of Newcastle, and other large towns. For 

 untold ages the Gulf Stream has been flowing as at present and 

 warming our western shores. The westerly winds blowing from it 

 across our islands were as charged with moisture ten thousand }^ears 

 ago as they are to-day. But these melanic forms did not exist half a 

 century since, and it would be idle to assume that they had been 

 brought into existence now by humidity which had failed in producing 

 such an effect in all these thousands of years. In the earlier part of 

 the paper (p. 17) ignoring " Natural Selection " and everything but 

 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 they are produced. Can we find one condition common 

 to all ? Mr. Tutt says humidity, but it is abundantly evident that 



