Haeckel immortalized his wife by giving her a living monument instead of one of marble or brass. He named for her one of his beloved medusæ, a fairy-like jellyfish, whose mass of long, trailing tentacles reminded him of his wife's blond hair. The Mitrocoma Ann? is described in his "Monograph on the Medusæ", published in 1864, and a note states that it was so named[10]
in memory of my dear, never-to-be-forgotten wife, Anna Sethe. If it is given to me to do something during my earthly pilgrimage for science and humanity, I owe it for the most part to the blessed influence of my gifted wife, who was torn from me by a premature end in 1864.
Three years afterwards he married again, Agnes Huschke, daughter of a Jena anatomist. They have three children, two daughters and a son, who has inherited his father's artistic talent and has devoted himself to art in Munich.
Haeckel's æsthetic taste is shown not merely in the thousands of paintings and drawings that fill his monographs, but especially in his "Art Forms of Nature", which consists of ten portfolios of large color plates depicting strange and beautiful creatures from all realms of animal life but particularly in little known lower forms, fishes, crustaceans, corals, radiolaria, diatoms, and desmids. Here are to be seen real gargoyles, more grotesque than a sculptor's unaided imagination can create. Here the designer and decorator can find hundreds of suggestive themes for almost any purpose, so they have no excuse for repeating the trite and traditional forms as they do.
A large part of these "art forms" Haeckel discovered in the course of his investigations of deep-sea life on the material gathered by the Challenger, which was commissioned by the British Government in 1872-1875 to explore the ocean. The results of this expedition, published in fifty large volumes, constituted the greatest contribution to oceanography that has ever been made. Haeckel contributed the volumes on the medusæ, the siphonophora, the keratosa, and the radiolaria. To the radiolaria Haeckel devoted ten years, 1877-1887, and described 4318 species and 739 genera, from the curiously complicated siliceous skeletons deposited on the bottom of the ocean by these minute one-celled creatures.
Although Haeckel's life was largely devoted to the closest study of the minutest forms of life, yet he never lost sight of the broader aspects of his science. It seems as though he felt the need of resting his eyes by raising them from the microscope and looking out of the window to focus on infinity. Haeckel is essentially a specialist with a fondness for generalization. He welcomed the change in the current of thought that set in at the close of the nineteenth century, the effort of the new century to get at the inner meaning of the mass of miscellaneous facts that the old century had heaped up. It was with intent to assist in this movement that he produced, at the age of sixty-five, his "Riddle of the Universe", intending this to be the final expression of his view of the world, a fragmentary sketch instead of the complete "System of Monistic Philosophy" which he had projected many years ago and could not now hope to complete. But five years later he supplemented this with a similar popular volume, "The Wonders of Life", in which he replies to certain criticisms and explains the biological principles on which his philosophy is based. This, unlike the "Riddle", was not composed at various intervals in the course of many years, but was written uninterruptedly during four months spent at Rapallo, on the Italian Riviera, when he was
stimulated by the constant sight of the blue Mediterranean, the countless inhabitants of which had, for fifty years, afforded such ample material for my biological studies; and my solitary walks in the wild gorges of the Ligurian Apennines and the moving spectacle of its forest-crowned altars, inspired me with a feeling of the unity of living nature—a feeling that only too easily fades away in the study of detail in the laboratory.
Professor Haeckel retired from active service as teacher and investigator in 1909 at the age of seventy-five. "Indeed I am wholly a child of the nineteenth century and with its close I draw the line under my life's work," he said, and the publication of "The Wonders of Life" in 1904 confirms rather than contradicts this, for it shows that he maintains his position altogether unshaken by the revolution that has taken place in philosophic thought. Like Herbert Spencer he lived to see a reaction against many of the opinions for which he fought most earnestly.
The nineteenth century was cocksure of so many things about which the twentieth century doubts. We are not so certain that, as Haeckel says, everything can be reduced to the motion of the atoms. The atom itself is crumbling, and as for motion, what is it? The ether in the reality of which Haeckel puts implicit faith is to us a doubtful, perhaps an unnecessary, hypothesis. Vitalism and teleology are coming back again into biology in new forms. Pluralism, not monism, is the fashion of the day, and some carry it almost to polytheism. Indeterminism finds more advocates nowadays than determinism. Haeckel makes the first law of thermodynamics (conservation of energy) one of the corner stones of his philosophy, but has little regard for the second (degradation of energy). Modern thought considers the second law more important than the first.[11]
And what shall we say about the "Law of Substance", which is Haeckel's contribution to the fundamental principles and which he apparently regards as of equal importance to the discoveries of Lavoisier and Mayer?[12] Speaking for myself, the reason I cannot accept it is because it is absolutely meaningless to me. We know what the law of the conservation of matter means. It means, among other things, that 12 pounds of carbon when burned make 44 pounds of carbon dioxid, which we may decompose and get back 12 pounds of carbon again. The law of the conservation of energy means, among other things, that when we burn 12 pounds of carbon we produce 135,305,600 foot pounds of energy. But what does it mean when we say that matter and energy, or body and spirit, are somehow the same substance? Have we said more than when we affirmed the two laws separately? Even if true, does it make a bit of difference to anybody or anything; or to put the query into the pragmatic form, can it be true if it does not make a bit of difference to anybody or anything? But we must bear in mind that the rigid application of this formula to many historic attempts to solve the "riddle of the universe" would leave less of them intact than in the case of Haeckel.