In consideration of Huxley's position before 1859, the most interesting feature of his zoölogical work is the gradual preparation that it was making in his mind for the doctrine of the _Origin_. He was like an engineer boring a tunnel through a mountain, but ignorant of how near he was to the pleasant valley on the other side; and, above all, ignorant how rapidly he was being met by a much more mighty excavation from the other side. To use what is perhaps a more exact simile: he was like a child with half the pieces of a puzzle-map, slowly linking them together as far as they would fit, and quite ignorant that presently the remaining half would suddenly be given him, and with almost no trouble would at once fit into the gaps he had necessarily left, and transform a meaningless pattern into a perfect and intelligible whole. Let us consider some of these map pieces. The ultimate picture was the conception of the whole world of life, past and present, as a single family tree growing up from the simplest possible roots, and gradually spreading out first into the two main branches of animals and plants, and then into the endless series of complicated ramifications that make up living and extinct animals and plants. Huxley was piecing together the scattered fragments, and gradually learning to see here and there whole branches, as yet separate at their lower ends, but in themselves shapely, and showing a general resemblance to one another in the gradual progression from simple to complex. The greatest of these branches that he had pieced together was the group of Medusæ and their allies, now known as Cœlenterates. He had formed similar branches for the Molluscs and minor branches for the Salps and Ascidians, and, in his general lectures on the whole animal kingdom, he had shadowed out the broad arrangement of the main divisions, or, as he called them, _types_. He had seen in each particular branch the clearest evidence of the laws of growth which had directed its development, and had realised that these laws of growth, consisting of gradual modifications of common typical structures, were identical in the different branches. He had taken clear hold of Von Baer's conception that the younger stages of different types were more alike than the adult stages, and here and there he had made comparisons between the younger stages or simplest forms of his different branches, and had shown that, without completely realising it, he was ready for the idea that just as the separate pieces could be arranged to form orderly branches, so the separate branches might come to be arranged as a single tree. And finally, in his lectures on "Protoplasm and Cells," and on the "Common Structure of the Animal and Plant Kingdoms," he had reached the conclusion that the two main divisions of the living world were formed of the same stuff, displayed in identical fashion the elementary functions of life, and were creatures of the same order.