Since this peculiar method of reproduction became known, in the thirties or forties, the medusæ were regarded as amongst the most interesting objects in the whole of zoology. They offered an extremely difficult task to the investigator who would care to take up the study of them. When Haeckel was with Johannes Müller in Heligoland in 1854 he made acquaintance with them for the first time. His artistic eye was caught with their beauty, as it was afterwards with the radiolaria. “Never shall I forget,” he says, “the delight with which, as a student of twenty years I gazed on the first _Tiara_ and _Irene_ [species of medusæ], and the first _Chrysaora_ and _Cyanea_, and endeavoured to reproduce their beautiful forms and colours.” His predilection for the medusæ never disappeared. At Nice in 1856 he met them again in the Mediterranean. Gegenbaur’s _Sketch of a Classification of the Medusæ_ provided his studies with a starting-point, just as Müller’s writings did afterwards for the radiolaria. At Naples and Messina he completed his mastery of them. When he had done with the radiolaria for the time after publishing the great monograph of 1862, the next task that loomed up on his horizon was the need for a “monograph on the medusæ.” It would be a long time, however, before he could complete the work in any fulness. A work of Agassiz that purported to do it, but, in his opinion, only confused the subject—he disliked both the Agassizs, father and son, and the father became one of his bitterest opponents on the Darwinian question—gave him a negative impulse to the study. He thought it would be best to deal with one family of the medusæ after another in separate monographs, as time permitted. The first of these essays appeared in 1864 and 1865, and dealt with what are known as the “snouted-medusæ” (_geryonidæ_). The first volume of the complete work was not published until fourteen years afterwards. If Haeckel had decided to work as a specialist he would have had material enough here to occupy him fully throughout the whole of the sixties, and even longer. The keen student of the radiolaria would be succeeded by the equally keen student of the medusæ. More folio volumes would have accumulated, with beautiful plates, such as only the technical student of zoology ever takes out of the library. His name, like that of his friend Gegenbaur almost, would never have reached the crowd. It was the influence of Darwin that prevented this. His attention was turned in another direction, and we begin to realise the full greatness of his power when we remember that he nevertheless continued with unfailing quality to publish such detailed studies as those on the medusæ.