e been
simply to account for certain conditions which resulted in such chemical
combination as would give rise to this physical basis of life. But now
that the simplest substance manifesting the phenomena of life is found
to be a machine, we can no longer find in chemical forces efficient
causes for its formation. Chemical forces and chemical affinity can
explain chemical compounds of any degree of complexity, but they cannot
explain the formation of machines. Machines are the result of forces of
an entirely different nature. Man can manufacture machines by taking
chemical compounds and putting them together into such relations that
their interaction will give certain results. Bits of iron and steel,
for instance, are put together to form a locomotive, but the action of
the locomotive depends, not upon the chemical forces which made the
steel, but upon the relation of the bits of steel to each other in the
machine. So far as we have had any experience, machines have been built
under the guidance of intelligence which adapts the parts to each other.
When therefore we find that the simplest life substance is a machine, we
are forced to ask what forces exist in nature which can in a similar way
build machines by the adjustment of parts to each other. But this topic
belongs to the second part of our subject, and must be for the present
postponed.

==Reaction against the Cell Doctrine.==--As the knowledge of cells which
we have outlined was slowly acquired, the conception of the cell passed
through various modifications. At first the cell wall was looked upon as
the fundamental part, but this idea soon gave place to the belief that
it was the protoplasm that was alive. Under the influence of this
thought the cell doctrine developed into something like the following:
The cell is simply a bit of protoplasm and is the unit of living matter.
The bodies of all larger animals and plants are made up of great numbers
of these units acting together, and the activities of the entire
organism are simply the sum of the activities of its cells. The organism
is thus simply the sum of the cells which compose it, and its activities
the sum of the activities of the individual cells. As more facts were
disclosed the idea changed slightly. The importance of the nucleus
became more and more forcibly impressed upon microscopists, and this
body came after a little into such prominence as to hide from view the
more familiar protoplasm. The marvell