om tidal phenomena to
the determination of the length of the day and of the time of
revolution of the moon, in past epochs of the history of the universe;
and the demonstration of the competency of the great secular changes,
known under the general name of the precession of the equinoxes, to
cause corresponding modifications in the climate of the two
hemispheres of our globe, have brought astronomy into intimate
relation with geology. Geology, in fact, proves that, in the course of
the past history of the earth, the climatic conditions of the same
region have been widely different, and seeks the explanation of this
important truth from the sister sciences. The facts that, in the
middle of the Tertiary epoch, evergreen trees abounded within the
arctic circle; and that, in the long subsequent Quaternary epoch, an
arctic climate, with its accompaniment of gigantic glaciers, obtained
in the northern hemisphere, as far south as Switzerland and Central
France, are as well established as any truths of science. But, whether
the explanation of these extreme variations in the mean temperature of
a great part of the northern hemisphere is to be sought in the
concomitant changes in the distribution of land and water surfaces of
which geology affords evidence, or in astronomical conditions, such as
those to which I have referred, is a question which must await its
answer from the science of the future.

[Sidenote: Biological sciences.]

[Sidenote: The 'cell theory.']

Turning now to the great steps in that progress which the biological
sciences have made since 1837, we are met, on the threshold of our
epoch, with perhaps the greatest of all--namely, the promulgation by
Schwann, in 1839, of the generalisation known as the 'cell theory,'
the application and extension of which by a host of subsequent
investigators has revolutionised morphology, development, and
physiology. Thanks to the immense series of labors thus inaugurated,
the following fundamental truths have been established.

[Sidenote: Fundamental truths established.]

All living bodies contain substances of closely similar physical and
chemical composition, which constitute the physical basis of life,
known as protoplasm. So far as our present knowledge goes, this takes
its origin only from pre-existing protoplasm.

All complex living bodies consist, at one period of their existence,
of an aggregate of minute portions of such substance, of similar
structure, called c