70 EXTEKNAL CONDITIONS OF VITAL ACTIVITY. 



to ivhich their surfaces are exposed. When birds of warm climates, 

 distinguished by the splendour of their plumage, are reared under an 

 artificial temperature in our own country, it is uniformly observed that 

 they are much longer in acquiring the hues characteristic of the adult ; 

 and that these are never so bright as when they have been produced 

 by the influence of the tropical sun. And it has been also remarked, 

 that if certain Insects (the Cockroach for example), which naturally 

 inhabit dark places, be reared in an entire seclusion from light, they 

 grow up almost as colourless as Plants that are made to vegetate under 

 similar circumstances. 



95. There is reason to believe that Light exercises an important in- 

 fluence on certain processes of development in Animals, as well as in 

 Plants. Thus, the appearance of Animalcules in infusions of decaying 

 organic matter is much retarded, if the vessel be altogether secluded 

 from it. The rapidity with which the small Entomostracous Crustacea 

 (Water-Fleas, &c.) of our pools, undergo their transformations, has been 

 found to be much influenced by the amount of light to which they are 

 exposed. And it has been ascertained that, if equal numbers of Silk- 

 worm's eggs be preserved in a dark room, and exposed to common day- 

 light, a much larger proportion of larvae are hatched from the latter 

 than from the former. The most striking proof of the influence of Light 

 on Animal development, however, is afforded by the experiments of Dr. 

 Edwards. He has shown that if Tadpoles be nourished with proper 

 food, and be exposed to the constantly renewed contact of water (so 

 that their respiration may be freely carried on, whilst they remain in 

 their fish-like condition), but be entirely deprived of light, their growth 

 continues, but their metamorphosis into the condition of air-breathing 

 animals is arrested, and they remain in the condition of gigantic tad- 

 poles. It is interesting to remark, that the Proteus anguineus, an ani- 

 mal which closely corresponds in its fully-developed form with the 

 transition stage between the Tadpole and the Frog, finds a congenial 

 abode in the dark lakes of the caverns of Styria and Carniola, and in 

 the underground caverns that connect them ; thus showing its adapta- 

 tion to a condition, which keeps down to the same standard the develop- 

 ment of an animal, that is empowered under other circumstances to ad- 

 vance beyond it. Numerous facts, collected from different sources, lead 

 to the belief that the healthy development of the Human body, and the 

 rapidity of its recovery from disease, are greatly influenced by the 

 amount of light to which it has been exposed. It has been observed, 

 on the one hand, that a remarkable freedom from deformity exists 

 amongst nations who wear very little clothing ; whilst, on the other, it 

 appears certain that an unusual tendency to deformity is to be found 

 among persons brought up in cellars or mines, or in dark and narrow 

 streets. Part of this difference is doubtless owing to the relative purity 

 of the atmosphere in the former case, and the want of ventilation in the 

 latter ; but other instances might be quoted, in which a marked variation 

 presented itself, under circumstances otherwise the same. Thus, it has 

 been stated by Sir A. Wylie (who was long at the head of the medical 

 staff in the Russian army), that the cases of disease on the dark side of 

 an extensive barrack at St. Petersburgh, have been uniformly, for many 





