es. Plainly, however, such questions were
inconceivable before about the middle of the nineteenth century.

This mechanical conception of living activity was carried even farther.
Under the lead of Huxley there arose in the seventh decade of the
century a view of life which reduced it to a pure mechanism. The
microscope had, at that time, just disclosed the universal presence in
living things of that wonderful substance, _protoplasm._ This material
appeared to be a homogeneous substance, and a chemical study showed it
to be made of chemical elements united in such a way as to show close
relation to albumens. It appeared to be somewhat more complex than
ordinary albumen, but it was looked upon as a definite chemical
compound, or, perhaps, as a simple mixture of compounds. Chemists had
shown that the properties of compounds vary with their composition, and
that the more complex the compound the more varied its properties. It
was a natural conception, therefore, that protoplasm was a complex
chemical compound, and that its vital properties were simply the
chemical properties resulting from its composition. Just as water
possesses the power of becoming solid at certain temperatures, so
protoplasm possesses the power of assimilating food and growing; and,
since we do not doubt that the properties of water are the result of its
chemical composition, so we may also assume that the vital properties of
protoplasm are the result of its chemical composition. It followed from
this conclusion that if chemists ever succeeded in manufacturing the
chemical compound, protoplasm, it would be alive. Vital phenomena were
thus reduced to chemical and mechanical problems.

These ideas arose shortly after the middle of the century, and have
dominated the development of biological science up to the present time.
It is evident that the aim of biological study must be to test these
conceptions and carry them out into details. The chemical and mechanical
laws of nature must be applied to vital phenomena in order to see
whether they can furnish a satisfactory explanation of life. Are the
laws and forces of chemistry sufficient to explain digestion? Are the
laws of electricity applicable to an understanding of nervous phenomena?
Are physical and chemical forces together sufficient to explain life?
Can the animal body be properly regarded as a machine controlled by
mechanical laws? Or, on the other hand, are there some phases of life
which the forc