INFLUENCE OF LIGHT ON PLANTS. 69 



seem as if they avoided the light, choosing the northern rather than the 

 southern sides of hedges, buildings, &c., for their residence; so that 

 the former often present a luxuriant growth of Cryptogamic vegetation, 

 whilst the latter are comparatively bare. It must not be supposed, 

 however, that they avoid light altogether, but only what is to them an 

 excessive degree of it. The avoidance of light seems to be much 

 stronger in the Fungi, which grow most luxuriantly in very dark 

 situations ; and the reason of this is probably to be found in the fact 

 that, like the germinating seed ( 89), they form rather than decom- 

 pose carbonic acid ; their food being supplied to them from the decay- 

 ing substances on which they grow ; and the rapid changes in their 

 tissues giving rise to a high amount of Respiration, a change exactly 

 the converse of that, on which, as we have seen, Light exerts such a 

 remarkable power. 



93. In regard to the agency of Light upon the functions of Animals, 

 comparatively little is certainly known. It is evident that the influence 

 it exerts on those chemical processes which constitute the first stage of 

 Vegetable nutrition, can have scarcely any place in Animals ; because they 

 do not perform any such acts of combination, but make use of the pro- 

 ducts already prepared for them by Plants. Hence, we do not find 

 that the surface of Animals undergoes that extension, for the purpose 

 of being exposed to the solar rays, which is so characteristic a feature 

 in the Vegetable fabric, and so important in its economy. Still there 

 can be no doubt, that the degree of exposure to light has a great 

 influence upon the colours of the Animal surface ; and here we seem to 

 have a manifestation of Chemical agency, analogous to that which gives 

 colour to the Vegetable surface. Thus it is a matter of familiar expe- 

 rience, that the influence of light upon the skin of many persons, causes 

 it to become spotted with brown freckles ; these freckles being aggre- 

 gations of brown pigment cells, which either owed their development to 

 the agency of light, or were enabled by that agency to perform a che- 

 mical transformation which they could not otherwise effect. In like 

 manner, the swarthy hue, which many persons acquire in warm cli- 

 mates, is due to a development of dark pigment-cells diffused through 

 the epidermis ( 229) ; and an increased development of the same kind 

 gives rise to the blackness of the Negro-skin. There can be no doubt 

 that the prolonged influence of light upon one generation after another, 

 tends to give a permanent character to this variety of hue ; which will 

 probably be more easily acquired, in proportion to the previously-exist- 

 ing tendency to that change. Thus it is well known that a colony of 

 Portuguese Jews, which settled at Tranquebar about three centuries 

 ago, and which has kept itself distinct from the surrounding tribes, can- 

 not now be distinguished as to colour from the native Hindoos. But it 

 is probable that a similar colony of fair-skinned Saxons would not, in 

 the same time, have acquired anything like the same depth of colour in 

 their skins. 



94. It can scarcely be questioned, that the brilliancy of colour which 

 is characteristic of many tribes of animals in tropical climates, especially 

 Birds and Insects, is in great part dependent, like the brightness of the 

 foliage and fruit of the same countries, upon the brightness of the light 



