VARIABILITY OF SPECIES. 117 



When, in 1873, the grave closed over Louis Agassiz, the 

 last great upholder of the constancy of species and of 

 miraculous creation, the dogma of the constancy of species 

 came to an end, and the contrary assumption — the assertion 

 that all the various species descend from common ancestral 

 forms — now no longer encounters serious difficulties. All 

 the elaborate inquiries as to the real nature of species, and 

 how it is possible that various species can proceed from 

 a single ancestral species, have now been brought to a 

 perfectly satisfactory close by the fact that the sharp de- 

 marcations between species and variety on the one side, 

 between species and genus on the other, have been entirely 

 set aside. I have given the analytical evidence of this in 

 my " Monograph on Chalk Sponges," 40 which appeared in 

 1872. In it I closelv examined the variations of all the 

 species of this small, but highly instructive group of animals, 

 and demonstrated in every instance the impossibility of 

 dogmatic distinctions of species. Just in proportion as the 

 systematizer takes the ideas of Genus, Species, and Varieties 

 in a wider or narrower sense, he distinguishes in the little 

 group of Chalk Sponges, either only a single genus with 

 3 species, or 3 genera with 21 species, or 21 genera witli 

 111 species, or 39 genera with 289 species, or even 113 

 genera with 591 species. But all these diverse forms are so 

 intimately connected by numerous transitions and inter- 

 mediate forms that the common descent of all the Chalk 

 Sponges from a single ancestral form, the Olynthus, can be 

 proved with certainty. 



I think I have thus given the analytical solution of the 

 problem of the Origin of Species, and have thus satisfied the 

 demands of those opponents of the Theory of Descent who 



