1890.] The Persistence of Plant and Animal Life. 523 
things would necessarily be by accretion ffom the elements which 
predominated in this soil and atmosphere. This is also what we 
observe. 
Some reasons for believing that life is simply one manifestation 
of force acting upon matter has been alluded to, but there are many 
other reasons. The demonstration of the correlation and conserva- 
tion of force, by Graham, Helmholtz, Meyer, Joule, Tyndall and 
others, in the early sixties marks an epoch in the science of 
physics. Since the date of this beautiful generalization it has 
been the practice to calculate all forms of force in terms of heat or 
heat-units ; and many experiments have shown that these heat- 
units were expended in carrying on the various life processes, 
precisely as they are in raising water into steam, and cooling the 
steam again into water by converting part of its heat into the 
mechanical motion of the parts of a machine. 
Regarding an animal as a machine, and its food as the fuel to 
drive this machine, an approximate calculation has been made of 
the directions in which the combustion (or assimilation) of the 
food is employed during the daily use of their organs by animals, 
and the calculation has been found to agree quite closely with 
observed facts. It may be safely predicated, therefore, that the 
force which builds up the plant or animal is calculable in so many 
heat-units expended to so much work of this kind performed ; i. e., 
to build an inch of sugar cane, as much force is required as 
would be represented by the burning of a given quantity of coal 
or wood, and the conversion of the heat thereby obtained into 
mechanical motion, etc. But the production of these heat-units 
must depend upon the ease with which certain elements or groups 
of elements can be broken up and formed into other groups; for 
this change, called chemical change, always results in the develop- 
ment of heat or its equivalent work. Keeping this fact in view, it 
is not difficult to understand why the bodies of plants and animals, 
which require for their very existence that these changes should 
be continually going on, shouldbe composed of groups of elements | 
easily broken up and re-formed, and of elements, too, which are 
known as combustibles, or those which greedily seek out and com- 
bine with oxygen. 
