384. Causes and Course of Organic Evolution 



other genera though starved in species at the present day, 

 and though greatly restricted in range, together show a dis- 

 tribution that much resembles Spongilla. For Europe and 

 South America, Asia, and Africa make up their continental 

 spread. 



Wlien we consider that the Flagellata are preponderatingly 

 fresh-water, also that Phalansterium and Proterospongia like- 

 ^^-ise are, a lacustrine origin for the entire group seems quite 

 likely. It would be difficult to explain the distribution of 

 Spongilla, in any other way, even if we grant that the gemmules 

 or pseudonavicellse are often distributed by other animals or 

 even by wind. 



But it must be acknowledged that on first view it seems 

 wholly unlikely that a large group like the sponges, which 

 includes 350 genera, nearly all of which are marine, should 

 have originated elsewhere than in the sea. So zoologists have 

 consistently and tacitly accepted such origin. But if this be 

 true it means that Spongilla has repeatedly adopted the habit 

 of migrating from the sea into inland waters over different 

 regions of the earth; or has very early passed from a marine 

 life in one or a few localities, and in the extensive changes 

 of the earth's surface has spread by eggs or by gemmules into 

 the world-wide areas where it is now found. But its com- 

 paratively simple skeletal structure, the continuity of the 

 surface and connective-tissue spongin fibers, the varjdng and 

 apparently diverse development of the embryo as traced by 

 Evans, as well as the origin and disposition of the tissue cells, 

 are all structural details that might favor the view of its being 

 one of many ancient fresh-water types, some of which have 

 died out, some have persisted in the other 10 or 11 genera, of 

 Spongillinse, while others have originated the complex, hetero- 

 genous, and abundant genera now found in sea waters. When 

 like evidence for the possible origin of other largely marine 

 groups is presented, it will be seen that much can be advanced 

 in favor of a lacustrine origin. About 350 living genera are 

 now recognized, of which 12 out of the 13 genera of Spongillinae 

 are alone fresh-water. 



