It no longer had any meaning to find that a special organ was adapted to
its conditions; but it was necessary to find out how it became adapted.
The difference in the attitude of these two points of view is
world-wide. The former fixes the attention upon the end, the latter upon
the means by which the end was attained; the former is what we sometimes
call _teleological_, the latter _scientific;_ the former was the
attitude of the study of animals and plants before the middle of this
century, the latter the spirit which actuates modern biology.

==The Mechanical Nature of Living Organisms.==--This new attitude forced
many new problems to the front. Foremost among them and fundamental to
them all were the questions as to the mechanical nature of living
organisms. The law of the correlation of force told that the various
forms of energy which appear around us--light, heat, electricity,
etc.--are all parts of one common store of energy and convertible into
each other. The question whether vital energy is in like manner
correlated with other forms of energy was now extremely significant.
Living forces had been considered as standing apart from the rest of
nature. _Vital force_, or _vitality_, had been thought of as something
distinct in itself; and that there was any measurable relation between
the powers of the living organism and the forces of heat and chemical
affinity was of course unthinkable before the formulation of the
doctrine of the correlation of forces. But as soon as that doctrine was
understood it began to appear at once that, to a certain extent at
least, the living body might be compared to a machine whose function is
simply to convert one kind of energy into another. A steam engine is fed
with fuel. In that fuel is a store of energy deposited there perhaps
centuries ago. The rays of the sun, shining on the world in earlier
ages, were seized upon by the growing plants and stored away in a
potential form in the wood which later became coal. This coal is placed