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 cells, each cell having its own life independent of
the others, though influenced by them.

All the morphological characters of animals and plants are the results
of the mode of multiplication, growth, and structural metamorphosis of