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