Evolution of Animals 383 



No one we believe will deny, on review of the ciliate and 

 flagellate Infusoria, that the entire present-day evidence is 

 in favor of their fresh-water origin. For, whether we regard 

 their richness in genera and species, their comparative struc- 

 ture, and their morphological relation to physiological stimuli, 

 the fresh-water forms fulfill all the requirements for such an 

 origin. Furthermore, they possess an added interest, since 

 Phalansterium, Codonodesmus, and Proterospongia are at once 

 fresh-water and are the only living types that readily connect 

 the Protozoa mth that aberrant and lowest group of the Meta- 

 zoa, namely, the Spongida, Porifera, or Parazoa, while other 

 genera are regarded by the writer as progenitors of the meta- 

 zoan Rotifera (p. 415). 



It would be inappropriate in the present volume to expand 

 at length on the possible geographical origin and present geo- 

 graphical distribution of the invertebrate Metazoa. Sollas 

 has given (loc. cit.) a full list of the groups and has ably argued 

 for a marine origin. A more detailed comparison however 

 of these groups not only weakens his contention; it seems to 

 be entirely negatived. But it should be observed, in following 

 out the inquiry, that where any group becomes preponder- 

 atingly marine, and specially semi-pelagic or pelagic, it may, 

 as in the cases of the Foraminifera and Radiolaria, branch 

 out into many generic and specific modifications, that impress 

 the mind more than do the steadily advancing, though possibly 

 less striking, forms that originated on land. 



As cases in evidence we might cite the Spongida or Pori- 

 fera, the Coelenterata, the Echinodermata, the Brachiopoda, 

 the Mollusca, and Cephalopoda. 



Amongst the Spongida or Porifera, the division Spongil- 

 linse includes eleven genera, all of the species of which are 

 fresh- water or rarely marine as in Lessepsia. Though referred 

 to the "heterogeneous" sub-group Demospongiese, they are 

 relatively simple in structure, while their distribution is most 

 suggestive. The fresh- water and the river sponges (S. lacustris 

 and S. fluviatilis) are the only living representatives of the 

 entire grouj) that are cosmopolitan, there being scarcely a region 

 of the world where such do not api)ear, if conditions during 

 recent geologic time have been fairly favorable. But the 



