science, and if we were to attempt to introduce these
in strict chronological order we should lose all sense of topical
continuity.

Our method has been to adopt a compromise, following the course of a
single science in each great epoch to a convenient stopping-point, and
then turning back to bring forward the story of another science. Thus,
for example, we tell the story of Copernicus and Galileo, bringing the
record of cosmical and mechanical progress down to about the middle
of the seventeenth century, before turning back to take up the
physiological progress of the fifteenth and sixteenth centuries. Once
the latter stream is entered, however, we follow it without interruption
to the time of Harvey and his contemporaries in the middle of the
seventeenth century, where we leave it to return to the field of
mechanics as exploited by the successors of Galileo, who were also the
predecessors and contemporaries of Newton.

In general, it will aid the reader to recall that, so far as
possible, we hold always to the same sequences of topical treatment of
contemporary events; as a rule we treat first the cosmical, then the
physical, then the biological sciences. The same order of treatment will
be held to in succeeding volumes.

Several of the very greatest of scientific generalizations are developed
in the period covered by the present book: for example, the Copernican
theory of the solar system, the true doctrine of planetary motions,
the laws of motion, the theory of the circulation of the blood, and the
Newtonian theory of gravitation. The labors of the investigators of the
early decades of the eighteenth century, terminating with Franklin's
discovery of the nature of lightning and with the Linnaean
classification of plants and animals, bring us to the close of our
second great epoch; or, to put it otherwise, to the threshold of the
modern period.

I. SCIENCE IN THE DARK AGE

An obvious distinction between the classical and mediaeval epochs may be
found in the fact that the former produced, whereas the latter failed
to produce, a few great thinkers in each generation who were imbued with
that scepticism which is the foundation of the investigating spirit; who
thought for themselves and supplied more or less rational explanations
of observed phenomena. Could we eliminate the work of some score or so
of classical observers and thinkers, the classical epoch would seem as
much a dark age as does the epoch that s