In this momentous duel of the two men who were regarded at the time as unquestionably the most distinguished scientists in Germany it seemed to most people for a time that Haeckel had gone off altogether into general and public questions with regard to the aim of research and philosophy. He seemed to lend colour to the belief as he published, in quick succession, a number of new popular lectures (_Cell-souls and Soul-cells_, 1878, and _The Origin and Evolution of the Sense-organs_, 1878), and at the same time published a collected volume of older and recent _Essays_ _on the Theory of Evolution_ (one part in 1878, a second in 1879, and a new and enlarged edition in 1902). As a matter of fact, we find him in these years occupied with a small but particularly well-lit field of his whole work. It was not merely that in a few years he buried himself in the primitive forests of Ceylon, in order to pursue his special studies far removed from all civilisation for months together. Just at this date appeared the great monograph on the medusæ, which he had at length concluded. The first volume (_The System of the Medusæ_, with 40 coloured plates) was published in 1879, and the second (_The Deep-sea Medusæ of the Challenger Expedition and the Organisms of the Medusæ_, with 32 plates) in 1881. And while these splendid volumes showed his academic colleagues that he had no mind to remain entirely on the outer battlements as a philosophic champion, he plunged up to the ears in a new special study of a range that would have made even the most enthusiastic specialist recoil.