siphonophores in the animal streams at Lanzarote, he entered upon a decisive study of the meaning of these real “social animals.” A social medusa has so great an appearance of unity that those who discovered it first did not believe it was a community, but a very complicated individual medusa. Vogt (1847) and Leuckart (1851) had denied this, and declared it to be a social group. But the controversy was still going on, as there was much difference of opinion as to the meaning of “social” and “state.” Haeckel now succeeded at Lanzarote in tracing for the first time the development of one of these siphonophores from the ovum. He was able to show that from the ovum only a single simple medusa is developed. This, then, becomes the parent of the community; it produces the rest of the members, not by a new sexual generation, but by budding out from itself, until the whole garland of connected individuals is ready to constitute the new over-individual, or the community. These luminous investigations were published three years afterwards (1869) in a work that was crowned by the Utrecht Society of Art and Science (_The Embryology of the Siphonophoræ_, with fourteen plates, published at Utrecht). But Haeckel returned time after time in later years to this group of animals with such great philosophic and zoological interest. When he had put before him in the eighties the whole of the siphonophores brought home by the splendid _Challenger_ expedition, he combined the material with the results of his own studies in a fine work, which was included (in English) in the publications of the _Challenger_ series at London, as the 28th volume of the _Zoology of the Challenger_, 1888. The voluminous work is illustrated with fifty masterly plates, some of them coloured, by Haeckel himself. The most important part of the text was also published in German at Jena, with the title, _System of the Siphonophoræ_. There is a good popular account of the siphonophore question in his lecture on “The Division of Labour in Nature and in Human Life” (1869). A few of these beautiful forms are also given on coloured plates in his illustrated work, _Art-forms in Nature_. Every thoughtful man ought, whatever his position is as regards Haeckel’s ideas, to glance at this material that he has so vigorously and clearly presented. While he was conducting this research into the embryonic development of the siphonophores, Haeckel made certain experiments on phenomena that have lately been made the subject of a special “experimental mechanical embryology” by some of his pupils, particularly Professor Roux, of Halle. He cut up siphonophore ova into several pieces at the commencement of their development, and saw an incomplete social medusa develop from each fragment. [Illustration: A SIPHONOPHORE (_Disconalia gastroblasta._) ]