elaborate elucidations, and have given
rise to a process of reasoning, the results of which can scarcely
yet be anticipated, but must bear in a very important degree upon
some of the most abstruse points of what may be called
transcendental physiology."

Many reasons make it difficult for us to realise, now, the singular
novelty and importance of Huxley's memoir on the Medusae. The first is
a reason which often prevents great discoveries in almost every
subject from receiving in after years their due respect. The years
that have passed since 1850 have seen not only the most amazing
progress in our knowledge of comparative anatomy, but almost a
revolution in the methods of studying it. Huxley's work has been
incorporated in the very body of science. A large number of later
investigators have advanced upon the lines he laid down; and just as
the superstructures of a great building conceal the foundations, so
later anatomical work, although it has only amplified and extended
Huxley's discoveries, has made them seem less striking to the modern
reader. The present writer, for instance, learned all that he knows of
anatomy in the last ten years, and until he turned to it for the
purpose of this volume he had never referred to Huxley's original
paper. When he did so, he found from beginning to end nothing that was
new to him, nothing that was strange: all the ideas in the memoir had
passed into the currency of knowledge and he had been taught them as
fundamental facts. It was only when he turned to the text-books of
anatomy and natural history current in Huxley's time that he was able
to realise how the conclusions of the young ship-surgeon struck the
Fellows and President of the Royal Society as luminous and
revolutionary ideas.

In the first half of the century, a conception of the animal kingdom
prevailed which was entirely different from our modern ideas. We know
now that all animals are bound together by the bond of a common
descent, and we seek in anatomy a clue to the degrees of relationship
existing among the different animals we know. We regard the animal
kingdom as a thicket of branches all springing from a common root.
Some of these spring straight up from the common root unconnected with
their fellows. Others branch repeatedly, and all the branches of the
same stem have features in common. What we see in the living world is
only the surface of the thicket, the tops of the twigs; and it is by