INFLUENCE OF HEAT ON PLANTS. 71 



years, in the proportion of three to one, to those on the side exposed to 

 strong light. And in one of the London Hospitals, with a long range 

 of frontage looking nearly due north and south, it has been observed 

 that residence in the south wards is much more conducive to the welfare 

 of the patients than in those on the north side of the building. 



96. These facts being kept in view, it is easy to perceive that there 

 must be differences among the various species of Animals, as among 

 those of Plants, in regard to the degree of light which is congenial to 

 them. Among the lowest tribes, in which no special organs of vision 

 exist, there is evidently a susceptibility to the influence of light, which 

 appears scarcely to deserve the name of sensibility, but which seems 

 rather analogous to that which is manifested by Plants ; thus among 

 those Polypes which are not fixed to particular spots, and amongst Ani- 

 malcules, there are some species which seek the light, and others which 

 shun it. And it appears from various observations upon the depths at 

 which marine animals are found, especially from the extensive series of 

 facts collected by Prof. E. Forbes,* that there are a series of zones, so 

 to speak, to be met with in descending from the surface towards the 

 bottom of the ocean, each of which is characterized by certain species 

 of animals peculiar to itself, whilst other species have a range through 

 two or more of the zones ; the extent of the range of depth, in each 

 species, bearing a close correspondence with the extent of its geographical 

 distribution. Now there can be no doubt, that the restriction of par- 

 ticular species to particular zones is due in great part to the degree of 

 pressure of the surrounding medium ; but there can be as little doubt, 

 that the variation in the degree of Light also exerts a most important 

 influence, the solar rays in their passage through sea water being subject 

 to a loss of one half for every seventeen feet. From the results of Prof. 

 Forbes's researches, it appears that no species of Invertebrated animals 

 habitually live at a greater depth than 300 fathoms ; and although Fishes 

 have been captured at a depth of from 500 to 600 fathoms, it is probable 

 that they had strayed from their usual abodes. 



2. Of Heat , as a Condition of Vital Activity. 



97. The most perfectly-organized body, supplied with all the other 

 conditions requisite for its activity, must remain completely inert, if it 

 do not receive a sufficient amount of Heat. The influence which this 

 agent ^ exerts upon living beings, is far more remarkable than its effects 

 upon inorganic matter ; although the latter are usually more obvious. 

 We are all familiar with its power of producing expansion, with the 

 liquefaction which is the consequence of its application to solids, with 

 the evaporation which it occasions in liquids, and with the enormous 

 repulsive force which it generates among the particles of vapours ; but 

 it is not until we look deeper than the surface, that we perceive how 

 immediate is the dependence of every action of Life upon this myste- 

 rious agent. The temporary or permanent loss of vitality, in parts of 

 the body subjected to extreme cold, is a " glaring instance" of the effect 



* Report on the Invertebrata of the ^gean Sea, in Transactions of British Associa- 

 tion, 1843. 



