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 forces of chemistry and physics cannot account for? Are there
limits to the application of natural law to explain life? Can there be
found something connected with living beings which is force but not
correlated with the ordinary forms of energy? Is there such a thing as
_vital energy_, or is the so-called vital force simply a name which we
have given to the peculiar manifestations of ordinary energy as shown in
the substance protoplasm? These are some of the questions that modern
biology is trying to answer, and it is the existence of such questions
which has made modern biology a new science. Such questions not only did
not, but could not, have arisen before the doctrines of the conservation
of energy and evolution had made their impression upon the thought of