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Calligenia ininiata, again, is normally pale red ; but a 

 variety was found at Lyndhurst in which the red was re- 

 placed by a bright lemon yellow. 



Among the mollusca, Tellina balthica has both yellow and 

 pink varieties ; and Helix nenioralis, as is well known, is 

 almost as frequently red as yellow, sometimes even com- 

 mencing life as red, and after a certain period producing a 

 new shell-growth of vivid yellow, as was the case in a 

 young specimen I received from Truro ; and it is notable 

 that the yellow portion corresponded with the thickening of 

 the shell and formation of a temporary lip after the first 

 period of growth. 



In the curious genus of toads called Boinbinator, we have 

 B. ignetis, with a red belly, and B. bonibineiis, with a yellow 

 belly, occurring in Europe ; while it would appear that in 

 Japan there occurs a species, similar in all respects to B. 

 igneiis, except that it has a yellow belly. Lastly, the red of 

 the chaffinch {Fringilla cceJebs) is sometimes replaced by 

 yellow ; and Mr. Jenner Weir told us here the other evening 

 that redpolls, when bred in confinement, lost the red colour 

 on their heads, and assumed yellow in its place. 



What, then, is the meaning of all this ? how is it that red 

 pigment is so readily replaced by yellow, and by no other 

 colour ? perhaps an analogy from inorganic nature may help 

 us. Take a solution of mercuric chloride or corrosive sub- 

 limate, and another of iodide of potassium, mix them care- 

 fully, putting not too much of the iodide, and you will get 

 a scarlet-red precipitate — the red iodide of mercury — dry 

 this carefully to a soft red powder, place some on a piece 

 of paper, and warm it gently over a lamp ; immediately it 

 becomes of a vivid yellow, losing all trace of the original 

 colour. Now scrape the yellow powder thus obtained with 

 the blade of a knife ; it at once resumes again its original 

 scarlet colour, and appears as before. 



What has happened is this : mercuric iodide has two 

 forms, one red, the other yellow, both identical in compo- 

 sition, but differing presumably in some arrangement of the 

 molecules at present unknown to us. The red form is the 



