THE DOCTRINE OF DESCENT. 15 



and others) ; and in the second place, the fact is equally 

 certain that descendants of one and the same species 

 which, according to the dogma of the old schools, could 

 always effect a fertile union under certain circumstances, 

 either cannot effect such a union or produce only barren 

 hybrids (the Porto- Santo rabbit, the different races of 

 horses, dogs, roses, hyacinths, &c. ; see " History of 

 Creation," vol. i., p. 146). 



For a certain proof that the conception of species 

 rests on a subjective abstraction and has a merely rela- 

 tive value like the conception of genus, family, order, 

 class, &c. no class of animals is of so much importance 

 as that of the Sponges. In it the fluctuating forms 

 vary with such unexampled indefiniteness and varia- 

 bility as to make all distinction of species quite illusory. 

 Oscar Schmidt has already pointed this out in the sili- 

 ceous sponges and keratose sponges ; and I, in my 

 monograph, in three volumes, on the Calcareous Sponges 

 (the result of five years of most accurate investigations 

 of this small animal group), have pointed out that we 

 may at pleasure distinguish 3, or 21, or 1 1 1, or 289, 

 or 591 different species. I also believe that I have 

 thus convincingly demonstrated how all these different 

 forms of the calcareous sponges may quite naturally, 

 and without any forcing, be traced to a single common 

 parent-form, the simple and not hypothetical, but 

 existing at this present day the simple Olynthus. 



