It may be said that Haeckel spent the rest of his life in filling in the outline he had sketched at the Stettin Congress of 1863, for, however detailed the work on which he was engaged, he never afterward lost sight of the guiding clew to the labyrinth of life evolution. We are here not concerned with the zoölogical studies on which his fame securely rests, but only with the philosophical views to which they led him. His convictions were very definitely established in early manhood, and he occupies to-day essentially the same point of view as fifty years ago. During this time his efforts have been increasingly directed toward reaching a wider audience. In 1866 he developed the fundamental principles of his monistic philosophy in the two large volumes of his "General Morphology of Organisms." This gained few readers outside the circle of savants, and little acceptance there. In 1868 he put his theory of evolution into more popular form in "The Natural History of Creation." This had an unusual sale for a book of its kind, but Haeckel was dissatisfied to see that the general public remained indifferent and unaffected by the new conceptions of the world and man arising from the discoveries of modern science. Worse still, he observed with alarm a rising tide of reactionary thought at the close of the century and a growing dominance of the clerical power in German politics. So he determined to make a final effort to influence his generation, an appeal to the court of last resort, the Cæsar of to-day, the people. He packed his science and philosophy into one volume of moderate size, filled in the chinks with obiter dicta, and published it in 1899 under the title of "The Riddle of the Universe." This time he hit the mark. The success of the book was immediate and amazing. An author of a detective tale or a Zenda romance might have envied him. Ten thousand copies were sold within a few months, one hundred thousand within a year, and by this time the sale of the German and English editions has doubtless passed the half million mark, not to speak of the fourteen other languages into which the book has been translated. Since a book like this usually has several readers for each copy, it is probable that those who have been directly reached by Haeckel within fifteen years must be numbered by the million. Besides this, of course, the spread of his views has been further extended by a similar volume, "The Wonders of Life", five years later, and by the widely circulated pamphlets of the Deutscher Monistenbund. Haeckels einheitliche Weltanschauung,[2] then, whatever one may think of it, is undeniably an important factor in the thought of to-day.
I found Professor Haeckel not altogether pleased that he owed his popular reputation to that one of his works in which he took the least pride. He seemed to hold it in almost as light esteem as his opponents and was frank in acknowledging its defects of style and content. "But," he said in substance to me, "I had set forth my philosophy with due dignity and order in my 'General Morphology' more than thirty years before and nobody read it. Nobody reads it now, even when they criticize my ideas. So what could I do but put them forth in a way that would secure attention?"
We must observe that to secure this wider audience he did not resort to any of the ordinary expedients, such as palliating unpopular views, skipping dry details, and avoiding technical terms. "The Riddle of the Universe" is not the sort of writing that goes by the name of "popular science" and that is commonly regarded as necessary to catch the attention and reach the understanding of the lay reader. Haeckel discusses questions of physiology, zoölogy, botany, paleontology, and astronomy, each in its own tongue, the bare facts stated without any poetic disguise or flowery adornment. Far from dodging long words when necessary, he invents them when unnecessary. Few men have done so much word coinage. In his work on the radiolaria alone he had to christen more than thirty-five hundred new species, two names apiece. So it is no wonder that when he comes to talking metaphysics and religion he sticks to the habit of making up his language as he goes.
In the case of other authors of this series I have had to distill the essence of their philosophy from the leaves of many volumes. I have had sometimes to translate poetry into prose and sometimes to piece together scattered suggestions and faint allusions into a coherent and compact doctrine. But in the case of Haeckel my task is easy, for nothing of the sort is necessary. He has himself expressed his views in succinct form and the plainest of language. He takes as much delight in creeds and dogmatic statements as any scholastic theologian, and he has the same implicit faith in formulas as capable of expressing all things in heaven and earth. One reason why his conflicts with the clergy have been so sharp and bitter is because he has much the same type of mind and uses similar language. Ordinarily, in the so-called warfare of religion and science, the adversaries revolve hopelessly around one another, like double stars, without ever coming into contact.
The most convenient formulation of Haeckel's philosophy for our purpose is that which he prepared as a sort of confession of faith for his lay church, the Monistenbund. It is here translated entire and for the most part literally, though in a somewhat condensed form.[3]
THE THIRTY THESES OF MONISM
I.—Theoretical Monism
1. Monistic Philosophy. The unitary conception of the world is based solely upon the solid ground of scientific knowledge acquired by human reason through critical experience.
2. Empiricism. This empirical knowledge is attained partly by sense observations on the external world and partly by conscious reflection on our mental internal world.