y other heat, it is as truly
correlated to the other forces as when it has a purely physical origin.
The amoeboid activity of a white blood corpuscle is stimulated within
certain limits by heat. Hatching of eggs and the germination of seeds
may be likewise hastened or retarded by access or deprivation of heat.
It was considerations such as these which led to the doctrine of
correlation of the vital and physical forces.

With respect to the muscular force exerted by an animal, it was supposed
that it was created by the animal. Dr. Frankland[36] says to this: "An
animal can no more generate an amount of force capable of moving a grain
of sand, than a stone can fall upwards or a locomotive drive a train
without fuel." As the amount of CO{2} exhaled by the lungs is increased
in the exact ratio of work done by the muscle, it cannot be doubted that
the actual force of the muscle is due to the converted potential energy
of the food. Since every exertion of a muscle and nerves involves the
death and decay of those tissues to a certain extent, as shown by the
excretions, Prof. Orton[37] has been led to say: "An animal begins to
die the moment it begins to live." "A muscle," says Barker,[38] "is like
a steam-engine, is a machine for converting the potential energy of
carbon into motion; but unlike a steam-engine, the muscle accomplishes
this conversion directly, the energy not passing through the
intermediate stages of heat. For this reason the muscle is the most
economical producer of mechanical force known." The muscles which give
the downward stroke of the wing of a bird are fastened to the
breastbone, and their power in proportion to the weight of the bird is
as 10,000 to 1. This great power is needed, for the air is 770 times
lighter than water; the hawk being able to travel 150 miles an hour.

The last of the so-called vital forces under consideration, is that
produced by the nerves and nervous centres. Barker says: "In the nerve
which stimulates a muscle to contract, this force is undeniably motion,
since it is propagated along this nerve from one extremity to the
other." This force has been likened unto electricity, the gray or
cellular matter being the battery, the white or fibrous matter the
conductors. Du Bois Reymond[39] has demonstrated that this force is not
electricity, though by showing that its velocity is only ninety-seven
feet a second. The velocity varies, though, in different animals; it is,
according to Pro