XVlil PREFACE. 
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 havea 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 haye given in my Mono- 
graph. 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 conse- 
quences 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 
