At the Easter of 1854 Haeckel returned from Würtzburg to Berlin. He was now twenty years old, and it was at this juncture that, to use his own phrase, the vast impression of Müller fell on him. A portrait of Müller still hangs over the desk in his study in the Zoological Institute at Jena. “If I ever become tired at my work,” he says, “I have only to look at it to get new strength.” The influence of the much older man, who, however, died at a far earlier age than Haeckel will do, only lasted for a short time. But Haeckel has preserved a memory of him that is only eclipsed by the memory of one other man—Darwin. Müller did not live to read Darwin’s decisive work, so that these two great ideals of Haeckel’s never crossed each other, either for good or evil. He himself felt that there was a pure evolution from one to the other in his mind. In the summer of 1854 he studied comparative anatomy under Müller, for which Kölliker had sufficiently prepared him. He has recorded his first impressions. “I soon got to know him personally, but I had so great a respect for him that I did not venture to approach him more closely. He gave me permission to work in the museum. I shall never forget the hours I spent there, drawing skulls, while he walked up and down, especially on Sunday afternoons. Often when he went past me I wanted to ask him something. I went up the step with beating heart and took hold of the bell, but returned without venturing to say anything.” Müller took some notice of the zealous young student. When the long vacation came round in August, and the master, following the new custom, packed up his bundle in order to spend two months on practical work by the sea, he allowed Haeckel to go with him. Müller’s son and the later Professor La Valette joined the party. They went to Heligoland. Müller taught his pupils his simple method of studying the living subject. There was no witchcraft in it, but it had had to be invented by some one. They put out to sea in a small boat. A little net of linen or fine gauze, with a wide opening and short body, was fastened on a pole. The mouth of the net was thrust directly under the surface or a little deeper, vertically to the surface, and the boat was slowly rowed forward. The contents of the filtered sea-water remained in the meshes of the net, and were from time to time emptied into a glass containing sea-water. “I shall never,” says Haeckel, “forget the astonishment with which I gazed for the first time on the swarm of transparent marine animals that Müller emptied out of his fine net into the glass vessel; the beautiful medley of graceful medusæ and iridescent ctenophores, arrow-like sagittæ and serpent-shaped tomopteris, the masses of copepods and schizopods, and the marine larvæ of worms and echinoderms.” Müller called these very fine and generally transparent creatures, of whose existence no one hitherto had had any idea, “pelagic sweepings” (from _pelagos_, the sea). More recently the