of the coral reefs.  He was not in any sense of the word a collecting naturalist.  The identification and naming of species interested him little.  What he cared for was, he tells us, “the architectural and engineering part of the business:  the working out of the wonderful unity of plan in the thousands and thousands of divers living constructions, and the modifications of similar apparatuses to serve different ends.”  And so, on the Rattlesnake, and in his work in continuation of the Rattlesnake investigations,—­which occupied most of his time for a few years after his return to London,—­there was gradually growing up in his mind a dim conception of the animal kingdom as a group of creatures, not built on half a dozen or more separate plans or types, each unconnected with the other, but as a varied set of modifications of a single type.

When Darwin set out on the Beagle, unlike Huxley, he was an enthusiastic collecting naturalist.  He had wandered from county to county in England adding new specimens to his collections of butterflies and beetles.  As the Beagle went round the world visiting remote islands, far from land in the centre of the waters, archipelagoes of islands crowding together, islands hugging the shore of continents, and the great continents of the old and new worlds, he continued to collect and to classify.  Gradually the resemblances and differences between the creatures inhabiting different parts of the earth began to strike him as exhibiting an orderly plan.  He saw that under apparently the same conditions of food and temperature and moisture, in different parts of the world the genera and species were different, and that they were most alike in regions 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.