528 EVOLUTION OF HEAT, LIGHT, AND ELECTRICITY. 



Animals are enabled to apply these compounds to the purposes of organiza- 

 tion ; and that, through the peculiar instruments thus constructed, those 

 various kinds of Vital Force are evolved, whose operations are so different 

 from any which we witness in the Inorganic world. Accordingly we ob- 

 serve that the " rate of life" in this larger proportion of the Animal king- 

 dom, is regulated, as in Plants, by the amount of Heat supplied to the or- 

 ganism from external sources; and that, when the external temperature is 

 reduced below a certain point, there is an entire cessation of all vital activ- 

 ity. But there are certain tribes, especially Birds and Mammals, which 

 possess the power of generating Heat within themselves, to such a degree 

 as to render the rate of their vital processes almost entirely independent of 

 external influences ; and there is probably no one species that can exercise 

 this power more effectually, and through a greater range of external con- 

 ditions, than Man is able to do. Of this we shall presently have evidence. 

 The evolution of Light, again, is by no means an unusual phenomenon 

 among the lower tribes of Animals; but where it does occur, it usually ap- 

 pears to have some special purpose, as is obvious enough in the case of the 

 glow-worm and other luminous Insects. But the luminosity which is occa- 

 sionally exhibited in Man must be regarded as an altogether abnormal 

 phenomenon, whose physiological interest arises out of the peculiarity of 

 the circumstances under which it presents itself. Of the degree in which 

 Electricity is generated in the living body, we know comparatively little. 

 There can be no doubt that a disturbance of Electric polarity takes place 

 in every action of Organic as well as of Inorganic Chemistry ; and thus 

 that every molecular change in the Animal as well as in the Vegetable or- 

 ganism must involve an alteration in its electric condition. But it would 

 seem that in the Animal body generally, these alterations are made to bal- 

 ance each other so exactly, that no considerable disturbance of the electric 

 equilibrium ordinarily takes place in the organism as a whole ; and it is 

 only in certain peculiar cases (as in the Electric Fishes) that a provision 

 exists for the generation of Electricity in considerable amount and inten- 

 sity, with a view to some special purpose. In the Human subject, however, 

 an extraordinary production of free electricity, as of Light, occasionally 

 presents itself; and this, taken in connection with other evidence, would 

 seem rather to indicate a departure, from the balance usually maintained 

 between the opposite electrical changes continually taking place, than to be 

 due to the introduction of any extraordinary sources of electric disturbance. 



2. Evolution of Heat. 



424. All the vital actions of the body of Man, as of that of " warm- 

 blooded " animals generally, require an elevated temperature as a condition 

 of their performance; and the high degree of constancy and i-egularity which 

 is observable in these actions, appears to depend in great degree upon the 

 provision which the organism contains within itself, for the maintenance of 

 that temperature at a fixed standard. This constancy and regularity are 

 most remarkably exhibited in the various periodical changes to which the 

 body is subject both in health and in disease ; the uniformity of whose re- 

 currence is due to a corresponding uniformity in the rate of vital action 

 taking place in the interval. Thus, as will be shown hereafter, the period 

 of parturition is in great degree determined by the maturation of the fu>tal 

 structures; and the uniformity of the time which this requires (like the cor- 

 responding uniformity in the period of development in the embryo bird) may 

 be fairly attributed to the regularity of the supply of Heat, which is the 

 power that especially determines the formative operations. For the periods 



