270 



DEFINITE ACTION OF THE 



Chap. XXUl 



" yellow colour, and on being plucked out, it is said, grow again of 

 " the same colour without any fresh operation." 



Bechstein^^ does not entertain any doubt that seclusion from 

 light affects, at least temporarily, the colours of cage-birds. 



It is well known that the shells of land-mollusca are affected by 

 the abundance of lime in different districts. Isidore Geoffroy Saint- 

 Hilaire ^^ gives the case of Helix lactea, which has recently been 

 carried from Spain to the South of France and to the Rio Plata, and 

 in both countries now presents a distinct appearance, but whether 

 this has resulted from food or climate is not known. With respect 

 to the common oyster, Mr. F. Buckland informs me that he can 

 generally distinguish the shells from different districts; young 

 oysters brought from Wales and laid down in beds where " natives " 

 are indigenous, in the short space of two months begin to assume 

 the " native " character. M. Costa^^ has recorded a much more re- 

 markable case of the same nature, namely, that young shells taken 

 from the shores of England and placed in the Mediterranean, at 

 once altered their manner ofgrowth and formed prominent diverging 

 rays, like those on the shells of the proper Mediterranean oyster. The 

 same individual shell, showing both forms of growth, was exhibited 

 before a society in Paris. Lastly, it is well known that caterpillars 

 fed on different food sometimes either themselves acquire a dilfei-ent 

 colour or produce moths differing in colour.^* 



It would be travelling beyond my proper limits here to discuss 

 how far organic beings in a state of nature are definitely modified 

 by changed conditions. In my ' Origin of Species ' I have given a 

 brief abstract of the facts bearing on this point, and have shown the 

 influence of light on the colours of birds, and of residence near the 

 sea on the lurid tints of insects, and on the succulency of plants. 

 Mr. Herbert Spencer*^ has recently discussed with much ability 

 this whole subject on general grounds. He argues, for instance, 

 that with all animals the external and internal tissues are differently 

 acted on by the surrounding conditions, and they invariably differ 

 in intimate structure. So again the upper and lower surfaces of 

 true leaves, as well as of stems and petioles, when these assume 



** ' Naturgeschichte der Stuben- 

 vogel,' 1840, s. 262,308. 



*^ ' Hist. Nat Gen.,' torn. iii. p. 

 402. 



« ' Bull, de la Soc. Imp. d'Accli- 

 mat.,' torn. viii. p. 351. 



** See an account of Mr. Gregson's 

 experiments on the Ahraxus grossu- 

 lariata, ' Proc. Entomolog. Soc.,' Jan. 

 6th, 1862: these experiments have 

 been confirmed by Mr. Greening, in 

 ' Proc. of the Northern Entomolog. 

 Soc.,' July 28th, 1862. For the etiects 

 of food on caterpillars, see a curious 

 account by M. Michely, in 'Bull, de 



la Soc. Imp. d'Acclimat.,' tom. viii. p. 

 663. For analogous facts from 

 Dahlbom on Hymenoptera, see West- 

 wood's ' Modern Class, of Insects,' vol. 

 ii. p. 98. See also Dr. L. Moller, 

 ' Die Abhangigkeit der Insecten,' 

 1867, s. 70. 



*^ 'The Principles of Biology,' vol. 

 ii., 1866. The present chapters were 

 written before I had read Mr. Her- 

 bert Spencer's work, so that I have 

 not been able to make so much use of 

 it as I should otherwise probably 

 have done. 



