on is, of course, in harmony
with the general trend of thought. To-day protoplasm is produced only
from other protoplasm; but, plainly, the first protoplasm on the earth
must have had a different origin. We must therefore next look for facts
which will enable us to understand its origin. We have seen that the
animal and plant machines have been built up from the simple cell as the
result of its powers acting under the ordinary conditions of nature.
Now, in accordance with this general line of thought, we shall be
compelled to assume that previous to the period of building machinery
which we have been considering, there was another period of machine
building during which this cell machine was built by certain natural
forces.

But here we are forced to stop, for nothing which we yet know gives even
a hint as to the method by which this machine was produced. We have,
however, seen that there are forces in nature efficient in building
machines, as well as those for producing chemical compounds; and this,
doubtless, suggests to us that there may be similar forces at work in
building protoplasm. If we can find natural forces by which the simplest
bit of living matter can be built up into a complicated machine like the
ox, with its many delicately adjusted parts, it is certainly natural to
imagine that the same forces may have built this simpler machine with
which we started. But such a conclusion is for a simple reason
impossible. We have seen that the essential factor in this machine
building is reproduction, with the correlated powers of variation and
heredity. Without these forces we could not have advanced in this
machine building at all. But these properties are themselves the result
of the machinery of protoplasm. We have no reason for thinking that this
property of reproduction can occur in any other object in nature except
this protoplasmic machine. Of course, then, if reproduction is the
result of the structure of protoplasm we can not use this factor in
explaining the origin of this protoplasm. The powers of the completed
machine can not be brought forward to account for its origin. Thus the
one fundamental factor for machine building is lacking, and if we are to
explain nature's method of producing protoplasm from simpler structures,
we must either suppose that the _parts_ of the cell are capable of
reproduction and subject to heredity, or we must look for some other
method. Such a road has however not yet been fou