gions between which
there was the most recent chance of migrations having taken place. In
the quietness of England, while Huxley was on the _Rattlesnake_,
Darwin was slowly working towards the explanation of all he had seen:
towards the conception that animals and plants had spread slowly from
common centres, becoming more and more different from each other as
they spread. He realised on his voyage that species had come into
existence by descent with modification, and before long he was to
publish to the world in the _Origin of Species_ a vast and convincing
bulk of evidence as to the actual fact of a common descent for all the
different existing organisms, and, in his theory of natural selection,
a reasonable explanation of how the fact of evolution had come about.
Darwin's greatest ally in bringing the new idea before the world was
Huxley, and Huxley was teaching himself the absolute unity of the
living world. The two men were dissimilar in tastes and temperament,
and they were at work on quite different sides of nature. When the
time came, Huxley, with his commanding knowledge of the structure of
animals, was ready to support Darwin and to illustrate and amplify his
arguments by a thousand anatomical proofs. It is a curious and
dramatic coincidence to realise that both men learned their very
different lessons under very similar circumstances in the tropical
seas of the Southern Hemisphere.

FOOTNOTES:

[Footnote B: _Narrative of the Voyage of H.M.S. "Rattlesnake_," by
John MacGillivray, F.R.G.S. 2 vols. T.W. Boone, London, 1852.]

[Footnote C: This sketch was reproduced and described in _Natural
Science_, vol. vii., p. 381, and is now reproduced here by the
courtesy of the proprietors.]

CHAPTER III

FLOATING CREATURES OF THE SEA

The Nature of Floating Life--Memoir on Medusae Accepted by the
Royal Society--Old and New Ideas of the Animal Kingdom--What
Huxley Discovered in Medusae--His Comparison of them with
Vertebrate Embryos.

As the _Rattlesnake_ sailed through the tropical seas Huxley came in
contact with the very peculiar and interesting inhabitants of the
surface of the sea, known now to naturalists as pelagic life or
"plankton." Although a poet has spoken of the "unvintageable sea," all
parts of the ocean surface teem with life. Sometimes, as in high
latitudes, the cold is so great that only the simplest microscopic
forms are able to maintain existence. In the tropics,