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most stable — probably the least complex — and is the usual 

 form of the salt, but under certain conditions — those of 

 heat — the yellow colour is assumed by a new arrange- 

 ment, which, being unstable, readily breaks up into the red 

 variety on friction. Suppose, now, that the red and yellow 

 animal pigments described above are merely two forms of 

 the same ; and that the yellow being the simpler, the red 

 develops only under certain conditions, which we do not 

 know, and has a tendency, under unfavourable circumstances, 

 even after it has become almost fixed by inheritance, to 

 degenerate into the yellow form, as in the case of Mr. Jenner 

 Weir's redpolls kept in confinement. 



I think this analogy is not too far-fetched, and the facts 

 seem to me to bear very strongly in its favour, though I 

 would have many more facts recorded and many more 

 careful experiments in breeding made before accepting this 

 or any other speculations on so difficult a question as proved. 

 One other important matter remains under the head of 

 colour-variation, that of melanism, or in less degree, darken- 

 ing and suffusion of the markings. This subject has been 

 a good deal under discussion of late, and various theories of 

 the cause of melanism have been brought forward. I, for 

 my part, attribute it rather to some atmospheric influences — 

 either directly to the effects of moisture in. the air, or to 

 something coincident with moisture. It seems possible, 

 though I am not sufficiently a meteorologist to say whether 

 it is so, that a moist atmosphere might hold in solution 

 gases which a dry atmosphere would destroy or not absorb. 

 If this is so, is it not conceivable that something of this 

 kind may have a hand in the production of melanism ? 

 When a room is crowded with human beings more than it 

 rightly should be, it is not unusual for each one there to go 

 away with a violent headache, produced by the exhalations 

 of the people in the room. Frequently, this is attributed to 

 the effects of the carbonic acid gas given out in breathing ; 

 but experiment has shown that the quantity of this gas is 

 by no means sufficient to produce the effects felt, and it is 

 therefore necessary to suppose that some gas exists in the 



