466 NOTES. 



contrary that these so-called glands are either illaminating organs, like 

 those detected by Giinther, or sensitive organs which have not yet been 

 developed into eyes. The genera in which he has discovered these sup- 

 posed 'pigmented glands' are Seopelui, Gonodoma, and Maurolimis. 

 Finally, there are two genera, Siemoptyu jn-oA. Arz/i/ropelecm, in which 

 the pigment-oeUs existing in corresponding spots in the body are said 

 to have a character between pigmented glands and true eyes. 



JVote 125, page 382. My brother, Georg Semper, has communicated 

 to me the following case, lately observed by him, of the adaptation of 

 an old species to the colour of new surroundings — or rather of its avail- 

 ing itself of it for protection. During the last ten years the weU-known 

 white-leaved variety of Acer negundo has been largely planted in 

 gardens in Hamburg, and since this the common white cabbage butterfly 

 has accustomed itself to settle by preference on this shrub. It is then 

 extremely diiBcult, as my brother informs me, to distinguish the butter- 

 flies as they sit on the leaves, their yellowish colour being lost in that 

 of the leaves. Here it is quite clear that the colour of the Pieris cannot 

 have been produced by selection, since it had the same characteristic 

 colouring long before the introduction of the white-leaved Maple. But 

 if now one or another species or variety could benefit by the similarity 

 of colouring wjiich has thus accidentally arisen, in the struggle for 

 existence, it would be at an advantage over any other species which was 

 by any cause disqualified from availing itself of this protection, and 

 thus the protective resemblance might occasion a selection among the 

 different forms. In this case, beyond a question, selection had abso- 

 lutely nothing to do with the origin of the protective colouring, and I 

 am convinced that in many cases, if not in all, the occurrence of pro- 

 tective resemblance is not to be explained by selection. Some very in- 

 teresting cases of protective mimicry are mentioned in Brazil,, the 

 Amazons, and tlie Coagt, by Herbert Smith (Sampson Low & Co.), 

 chap. vii. 



Note 126, j)age 387. The pigment-forming matter — chromogene, as 

 it is called — is conveyed by the blood to every organ in the body. K 

 depends on local conditions whether it is in some places deposited in 

 abundance and in others not at all. Consequeijtly the primary distri- 

 bution of colour depends on the structture of the organ or of that portion 

 of the skin where such a deposit normally takes place. Examples to 

 prove this are absolutely innumerable ; they may be found in almost 

 every animal. The gay colours of many shells — both bivalve and 

 univalve— are in great measure produced by the deposition of pigment in 

 the external organic skin, which covers the calcareous portion of the 

 shell ; the pigment itself is elaborated by glands which exist exclusively 

 in the margin of the mantle. It is according to the regularity of the 

 arrangement of these pigment-glands and the interruption in the exercise 

 of their functions that certain patterns and colours occur in the shells^ 



