growing out from a single “stock” or “cormus”—forms which until now have been regarded by systematists, not only as belonging to different species, but even to different genera. Fig. 10 in the frontispiece represents such a composite stock. This solid and tangible piece of evidence in favour of the common descent of different species ought, one would think, to satisfy the most determined sceptic! In point of fact, I have a right to expect of my opponents that they shall carefully consider the “exact empirical proof” here brought forward for them, as they have so eagerly demanded. The opponents of the doctrine of filiation, who have too little power of weighing evidence, or possess too little knowledge to appreciate the overpowering weight of proof afforded by the synthetical argument (comparative anatomy, ontogeny, taxonomy, etc.), may yet be able to follow me along the path of analytical proof, and attempt to upset the conclusion as to the common origin of all species of all Calcareous Sponges which I have given in my Monograph. I must, however, repeat that this conclusion is based on the most minute investigation of an extraordinarily rich mass of material,—that it is securely established by thousands of the most careful microscopical observations, measurements, and comparisons of every single part, and that thousands of collected microscopic preparations render, at any moment, the most searching criticism of my results confirmatory of their correctness. One may hope, then, that opponents will endeavour to confront me on the ground of this “exact empiricism,” instead of trying to damn my “nature-philosophical speculations.” One may hope that they will endeavour to bring forward some evidence to show that the latter do not follow as the legitimate consequences of the former. May they, however, spare me the empty—though by even respectable naturalists oft-repeated—phrase, that the monistic nature-philosophy, as expounded in the “General Morphology,” and in the “History of Creation,” is wanting in actual proofs. The proofs are there. Of course those who turn their eyes away from them will not see them. Precisely that “exact” form of analytical proof which the opponents of the descent theory demand is to be found, by anybody who wishes to find it, in the “Monograph of the Calcareous Sponges.” ERNST HEINRICH HAECKEL. _Jena, June 24th, 1873._ NOTE.