140 Wisconsin Academy of /Sciences, Arts, and Letters. 



origin of its species. This particular animal group is especially 

 fitted for the analytical solution of the species problem, because 

 it presents exceedingly simple conditions of organization ; because 

 in it, the morphological conditions possess a greatly superior, and 

 the physiological conditions are inferior, in part, and because all 

 species of the CalcispongiEe are remarkable for the fluidity and 

 plasticity of their form. With a view to these facts, I made two 

 journeys to the sea-coast (1869 to Norway ; 1871 to Dalmatia), 

 in order to study as large a number of individuals as possible, in 

 their natural circumstances, and to collect specimens for compari- 

 sons. Of many species, I compared several hundred individuals 

 in the most careful way. I examined with the microscope, and 

 measured in the most accurate manner, the details of form of all the 

 species. As the final result of these exhaustive and almost end- 

 less examinations and measurements, it appeared that ' good 

 species,' in the ordinary dogmatic sense of the systematists, have 

 no existence at all among the Calcareous Sponges ; that the most 

 different forms are connected, one with another, b}?- numberless 

 gradational transition forms ; and that all the different species of 

 Calcareous Sponges are derived from a single exceedingly simple 

 ancestral form, the Olynthus. If we take for the limitation of 

 genera and species, an average standard, derived from the actual 

 practice of naturalists, and apply this to the whole of the Calcare- 

 ous Sponges at present known, we can distinguish about 21 genera 

 with 111 species. I have however, shown that we may draw up, 

 in addition to this, another systematic arrangement, which gives 

 29 genera and 289 species. A systematist, who gives a more lim- 

 ited extension to the ideal species, might arrange the same series 

 of forms in 43 genera, and 881 species, or even in 118 genera and 

 690 species ; another systematist, on the other hand, who takes a 

 wider limit for the abstract " species," would use in arranging the 

 same series of forms, only 3 genera, with 21 species, or might even 

 satisfy himself with 2 genera and 7 species. This appears to be 

 so arbitrary a matter, on account of endless varieties and transi- 

 tional forms in this group, that their number is entirely left to the 

 subjective taste of the individual systematist." 



"In point of fact," he continues, " I have a right to expect of 



