648 



cannot, I think, be taken as evidence against the correctness of my 

 observations. On the other hand, the results of my experiments 

 may be found to put a new construction upon the facts observed by 

 Prof. Forbes. He discovered that increased depth of sea corre- 

 sponds with diminished light, and that both of these conditions 

 again correspond with peculiar changes in colour, and ultimately 

 with loss of colour in the shells inhabiting these depths ; but there 

 is no evidence to show that these colourless shells have developed 

 any materials capable of manifesting colour after exposure to the 

 influence of light ; whereas my own and other experiments show 

 that the etiolated stalks and leaves of plants speedily manifest the 

 characteristic colour of the chlorophyl if placed in the sun's rays. 



So far, therefore, as our present knowledge on the subject justi- 

 fies any conclusion, the varieties of colour and the absence of colour 

 in the mollusks are physiologically separated from the phenomena of 

 etiolation in plants, and may be placed in the same category as the 

 varieties of colour and the absence of colour in the corollas of 

 flowers, which depend upon the development of materials having 

 certain optical properties. 



The beautiful facts observed by Prof. Forbes, instead of being 

 regarded as the consequence of imperfect exposure to light, must, I 

 think, take rank with the phenomena of coloration observed through- 

 out the animal kingdom, such as the peculiar markings of reptiles, 

 birds, and wild animals, according to their different habitats and 

 modes of life ; the colours of the upper and lower surfaces of fish, 

 and the like, which cannot be shown to depend upon the exposure 

 or non-exposure to light with which they frequently, but not always, 

 coincide. These facts appear only to form a part of the vast and 

 perfect plan of creation, in which everything that exists is suited in 

 every particular to the conditions of its existence ; thus, those mol- 

 lusks which are designed to inhabit depths scarcely permeable to 

 light, can have no need, and hence have no provision, for elements, to 

 the manifestation of which light is an essential condition. 



