These curious discussions did not seriously interfere with the success of the book. In thousands and thousands of minds, in 1868, this little work proved the grain of seed that led on in time to serious thought. From that time onward Haeckel knew that he had not only scientific colleagues and academic pupils, but a crowd of followers. When he made an excursion into the northern part of the Sahara, as far as the first oasis, twenty-two years afterwards, he met an artist there. They talked philosophy, and the man, not knowing Haeckel, naïvely recommended him to study the History of Creation as likely to give him most help. The little incident shows us something of the great pioneer work done by the volume, something of its spiritual circumnavigation of the globe.
Thus the spiritual nucleus of the General Morphology is introduced, with great ability, to a much wider circle than Haeckel had dreamed of when he gave the Morphology to his colleagues. But the agitation gradually spread into academic circles. On the whole the Darwinian ideas pressed in everywhere by their own irresistible weight. Haeckel’s more particular concern, however, was to secure the recognition of one single point in the larger group of ideas—the great biogenetic law. This was for many years the pivot on which almost all the discussions with him and about him turned.
He himself did not at first conceive his law as a matter of controversy, but as a method that must be brought into a position of practical utility. An opportunity to do this arose immediately.
While he was at Lanzarote he began to take an interest in a second group of lowly animals besides the siphonophores, namely the sponges. When the general reader hears the word “sponge” he must modify his ordinary ideas a little. In the present instance he must not think of the plants, belonging to the fungi-group, such as the morel and cognate forms, that are often called “sponges” in common parlance. He must think rather of the sponge he uses in his bath. The bath-sponge is a structure made up of very tough, elastic, horny fibres. This structure is originally the skeleton, as it were, of certain animals that are known as “sponge-animals” or, briefly, sponges; they have nothing to do with the spongy mushrooms I spoke of. At the same time these socially-living sponges are such curious creatures that it was disputed for a long time whether they were real animals or not. There was a second controversy in regard to them as to where the “individual” began—what was a single animal, and what a co-operative colony of animals. The latter point alone would have been enough to direct Haeckel’s attention to this group after he had, in the case of the siphonophores, gone so deeply into the mystery of combined individuals, forming a new “state-individual.” His own opinion eventually was that as a matter of fact in the majority of cases the whole sponge is a stock or colony of separate sponge individuals closely connected together. They had not, indeed, anything like the ingenious method of division of labour that we find in the social medusæ; in fact, the sponges are in all respects much more lowly organised animals than the medusæ. But they were certainly true animals. And in the middle of his efforts to prove this Haeckel travelled into an entirely new field of research, lying far beyond the theory of individuality.
As there is an enormous number of different sponges, he had confined his studies from the first to a single group of them that might be taken as typical. He chose the calcispongiæ (calcareous sponges), which had been the least studied up to that time. As the name obviously implies, these sponges form their internal framework or skeleton, not of elastic horny fibres like the common bath-sponge, but of solid calcareous needles or spines. They secrete these out of the soft substance of their bodies just as the radiolaria do their pretty siliceous houses. Haeckel was engaged for five years, from 1867 to 1872, in a profound and careful study of the natural history of the calcispongiæ. Then he published the results in his Monograph on the Calcispongiæ, consisting of two volumes of text and an atlas of sixty fine plates.
The first result was that the calcispongiæ afforded a splendid proof of the impossibility of drawing sharp limits between species in the perpetually developing animal world. In their case the different varieties passed constantly out of each other and back into each other in a way that would have made a classifier of the old type distracted. But Haeckel had travelled far beyond the position of his boyhood, when he had timorously concealed the bad species that would not fit into the system. He said humorously that in the case of the calcispongiæ you had the choice of distinguishing one genus with three species, or three genera with 239 species, or 113 genera with 591 species. All this confusion was saved by the Darwinian idea of not setting up absolutely rigid classes, families, genera, and species. But even this was not yet the essential point.
Ernst Haeckel, 1874.
As he had done in the case of the siphonophores, Haeckel endeavoured to derive as much information as possible from the “ontogeny,” or embryonic development, of the calcispongiæ. He established in some cases, it seemed to him, that a single calcisponge-individual at first and up to a certain stage developed from the ovum in the same way as a medusa or a coral or an anemone. The fertilised ovum, a single cell, divided into two cells, then several, and at last formed a whole cluster of cells. In this cluster the cells arranged themselves at the surface, and left a hollow cavity within. Then two layers of cells were formed, like a double skin, in the wall of this vesicle, and an opening was left at one spot in the wall of it. Thus we got a free-swimming embryo, with a mouth, an external skin, and an internal digestive skin or membrane. Then the creature attaches itself to the floor of the sea and becomes a real sponge, partly by developing along its characteristic lines, and partly (in most cases) by producing other sponges from itself in the form of buds, like the siphonophore, and so forming an elaborate colony, to which we give collectively the title of “a sponge.” These facts led to the following reflections.