XVlll 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 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 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 



