organs for the purpose; waste matter remains in the flesh and blood of
dead animals. In plants are found a large number of powerful volatile
oils, alkaloids, bitter resins, etc. Many of these are, in all
probability, excretory products of no assimilative value to the plant.
Certain volatile oils may attract insects, and in obtaining nectar from
flowers insects assist fertilisation. Agreeable volatile oils and
flavouring substances in fruits attract birds and animals. The eating of
the fruits cause the seeds, which are uninjured by passing through the
digestive system, to be disseminated over wide areas to the advantage of
the plant species. On the other hand, nauseous and poisonous alkaloids,
oils, resins, etc., serve as a protection against the attacks of browsing
animals, birds, caterpillars, snails, etc. These nauseous substances are
most abundant in the bark, husk, skin and outer parts. It is commonly
supposed that the food on which each animal, including man, subsists, is
especially produced by Nature for the purpose. This is an error, for each
species of plant and animal lives for itself alone, and protects itself,
with more or less success, against destruction by its competitors and
enemies. Each species of animal selects from its surroundings such food as
is most suitable. Such food may not be theoretically perfect; that is, it
may not contain the maximum of nourishment free from innutritious matter;
but during the long period of evolution, each species of animal has become
possessed of organs suited to its environment. If to such animals be given
food containing less indigestible matter, or food which is more readily
digested by laboratory tests made independently of the living animal,
their digestive system will be thrown out of gear, become clogged up or
refuse to work properly, just as the furnace of a steam boiler, made to
burn coal, will act badly with wood or petroleum. Many scientific men have
overlooked this fact, and have endeavoured to produce food substances for
general consumption, in the most concentrated and soluble form, thinking
such food would be more easily assimilated.