Philosophical Conceptions of Life. 237 
investigation—no logical difficulty of a radical kind. In a 
general way I recognize that the matter of which living beings 
are composed is built up of elementary substances belonging 
to the inorganic world, and that it consists of atoms possessed 
of the very same properties and obedient to the very same 
laws as like atoms in inorganic bodies. Yet 1 confess I find 
in all this no reason for denying the existence of a vital prin- 
ciple; only I do not figure this principle in my mind as a 
hostile power interfering in any way with the chemical ten- 
dencies of the atoms present; I liken its operations rather to 
those of the chemist in his laboratory who obtains the results 
he needs only on the condition of most rigid obedience to 
chemical laws. 
Intimately associated with some of the chemical processes 
just enumerated are those chemical processes of respiration in 
which the chemical affinities of the oxygen of the atmosphere 
are directly or indirectly the means of promoting tissue meta- 
morphosis, as well as of reducing at once to simpler forms 
some portion of the various complex substances derived from 
the food. These chemical processes are undoubtedly the chief 
original sources of the heat and mechanical power manifested 
by animals. Of course they receive heat also from without 
by conduction and radiation; but this is a small matter to the 
heat generated within them ; of course, too, mechanical power 
is continually transformed into heat within the body of ani- 
mals; but this neither increases nor diminishes the total 
amount of energy liberated. 
I yield my hearty assent to that modern scientific induc- 
tion * which sees in the potential energy of the complex 
chemical compounds supplied to animals by their food the 
essential source of all the actual energy of the body, whether 
manifested in the form of heat or work. In a general way 
the reduction of these complex chemical compounds by oxida- 
tion into the much simpler ones, urea, carbon dioxide, and 
water, is the means by which potential is converted into actual 
energy. In the case of plants, too, the source of any little 
heat that may be developed under special conditions, and of 
such sluggish motions as actually occur, is doubtless to be 
found in the reduction to simpler combinations by oxidation 
of a part of the organic matter already formed. ‘The chief 
function of the vegetable world, however, is to build up, by 
means of the solar energy, those complex and unstable 
* First taught by J. R. Mayer, ‘Die organische Bewegung in ihrem 
Zusammenhange mit dem Stoffwechsel: Kin Beitrag zur Naturkunde ' 
(Heilbronn, 1845). 
