THE BRITISH NATURALIST. 



[August 



explain these discrepancies by a difference in rain-fall, I however 

 point them out to show how very far from completely proven is this 

 association of melanic forms and smoke polluted skies. However, 

 dismissing such doubts for the moment, let us accept as correct the 

 definition of Melanism No. 2, as given above, and let us examine for 

 a moment Mr. Tutt's theories and consider how they fit the facts of 

 the case as admitted by himself. 



Careful perusal of " Melanism and Melanochroism " certainly 

 leaves the reader under the impression that Mr. Tutt ascribes all 

 melanism of every kind principally to humidity, and his reply to Mr. 

 Robson's criticisms, 1 a reply in which he has allowed himself to adopt 

 a tone happily rare in Entomological dialectics, does not remove that 

 impression, although its evident animus deprives it of any argumenta- 

 tive value. 



To the potent influence of humidity then does Mr. Tutt refer 

 ultimately all melanic phenomena, but in trying to prove this, he, to 

 my mind, in the first place, insufficiently discriminates between the 

 two classes of melanism which he has to explain, and further confuses 

 his argument, unintentionally I think, by the introduction of two 

 principles whose exact bearing and character he appears to mis- 

 understand, heredity (p. 30 et seq.) and natural selection (passim). Now, 

 heredity is strictly only that principle, as to the exact cause 

 of which biologists are by no means agreed, but which compels 

 offspring to resemble parents. If you have a preponderating melanic 

 tendency in the parents the offspring will inherit such tendency, but 

 it is obvious that such a statement and such a principle explain the 

 perpetuation only, not the origin of melanism. 



Again, it is of the utmost importance in biological, discussion to be 

 quite clear as to the exact meaning of Natural Selection. 



Natural Selection can never originate, it merely carries into effect, 

 it is the machinery, not the motive power, and, of course, the principle 

 of heredity is included in the term Natural Selection. 



Mr, Tutt appears to me again to have obscured his reasoning by 

 using these terms inexactly and unmeaningly. 2 Now, if he had simply 

 stated that among other possible causes of melanism he believed 

 humidity to be the most effective, inasmuch as given a humid 

 en\ T iroment, melanism would for various reasons be of advantage to 

 the species affected by it, that heredity might have afforded the 

 possible initial cumulative variation, and that natural selection was 

 the process by which the end had been achieved. Such a statement 

 would at least have enabled his readers to exactly understand what 

 his theory might be. I believe we owe the want of such lucidity to 



1. " Entomologist's Record," May, 1893. 



2. e.g. Answer to Mr. Robson, " Record," No. 5, Vol. IV., page 1, line 20., et seq. 



