of the steps in this history
of machine building. The history can be traced by the study of these
samples just as the history of any machine might be traced from a study
of the models in a patent office. One might very easily trace, with most
strict accuracy and minute detail, the history of the printing machine
from the models which are preserved in the patent offices and elsewhere.
So is it with the history of the living machine. To be sure, the history
is rather incomplete and at times difficult to read. Many a period in
the development has left no samples for our inspection and must be
interpreted in our history between what went before and what comes
after. Many of the machines, especially the early ones, were made of
such fragile material that they could not be preserved in the rocks. In
many a case, too, the rocks in which the specimens were deposited have
been subjected to such a variety of heatings and pressures, that they
have been twisted out of shape and even crushed out of recognizable
form. But in spite of this the record is showing itself more complete
each year. Our paleontologists are opening layer after layer of these
rocks, and thus examining each year new pages in nature's history. The
more recent epochs in the history have been already read with almost
historic accuracy. From them we have learned in great detail how the
finishing touches were given to these machines, and are able to trace
with accuracy how the somewhat more generalized forms of earlier days
were changed to produce our modern animals.

This fossil record has given us our best knowledge of the course by
which the present living world has been brought into its existing
condition. But its accuracy is largely confined to the recent periods.
Of the very early history fossils tell us little or nothing. All the
early rocks, which we may believe were formed during the period when the
first steps in this machine building were taken, have been so changed by
heat and pressure that whatever specimens they may have originally
contained have been crushed out of shape. Furthermore, the earliest
organisms had no hard skeletons, and it was not until living beings had
developed far enough to have hard parts that it was possible for them to
leave traces of themselves in the rocks. Hence, so far as concerns this
earliest history, we can get no record of it in the rocks.

==Embryological.==--But here comes in another source of evidence which
helps to fi