number of structural parts adapted to each other in such a way as to
work together in the form of an intricate mechanism. The whole problem
would then be changed. To explain such a machine we could no longer call
upon chemical forces. Chemical affinity is adequate to the explanation
of chemical compounds however complicated, but it cannot offer any
explanation for the adaptation of parts which make a machine. The
problem of the origin of the simplest form of life would then be no
longer one of chemical but one of mechanical evolution. It is plain then
that the question of whether we can attribute the properties of the
simplest type of life to chemical composition or to mechanical structure
is more than a formal one.

==The Discovery of Cells.==--It is difficult for us to-day to have any
adequate idea of the wonderful flood of light that was thrown upon
scientific and philosophical study by the discoveries which are grouped
around the terms cells and protoplasm. Cells and protoplasm have become
so thoroughly a part of modern biology that we can hardly picture to
ourselves the vagueness of knowledge before these facts were recognized.
Perhaps a somewhat crude comparison will illustrate the relation which
the discovery of cells had to the study of life.

Imagine for a moment, some intelligent being located on the moon and
trying to study the phenomena on the earth's surface. Suppose that he is
provided with a telescope sufficiently powerful to disclose moderately
large objects on the earth, but not smaller ones. He would see cities in
various parts of the world with wide differences in appearance, size,
and shape. He would see railroad trains on the earth rushing to and fro.
He would see new cities arising and old ones increasing in size, and we
may imagine him speculating as to their method of origin and the reasons
why they adopt this or that shape. But in spite of his most acute
observations and his most ingenious speculation, he could never
understand the real significance of the cities, since he is not
acquainted with the actual living unit. Imagine now, if you will, that
this supramundane observer invents a telescope which enables him to
perceive more minute objects and thus discovers human beings. What a
complete revolution this would make in his knowledge of mundane affairs!
We can imagine how rapidly discovery would follow discovery; how it
would be found that it was the human beings that build the houses,
const