272 PROCEEDINGS OF THE ACADEMY OF [1887. 



CONCLUSION. 



In closing this (third) Monograph of the fresh-water sponges, it is 

 with the consciousness that the work of classification occupies a very 

 humble place among biological efforts and that all systems must of 

 necessity be tentative and temporary, soon to be superseded by oth- 

 ers, the results of a larger knowledge, gained by the contemplation 

 of a wider horizon. Even so, it has its value in helping forward 

 this very result. 



Some points in the present seemingly narrow field of scientfic la- 

 bor, worthy of the thought and study of future students have already 

 been suggested, such as the necessity of gemmules in fresh water as 

 distinguished from marine sponges ; the process of their formation ; 

 their function and the means by which that end is attained ; the law 

 of variation in the quantity and character of the enveloping crust, 

 and the time and mode of formation of the embedded armature ; — 

 all have yet to be conclusively studied. Other questions of a more 

 limited character occur, in the search for the line of derivation that 

 must be supposed to run through all the genera and species ; and in 

 the association, apparently indicated amongst otherwise dissimilar 

 species, by the presence in them of correspondent forms, such as the 

 birotulate dermals found in certain Spongillas and Meyenias and 

 the more frequent recurrence in several genera, of acerate dermals 

 with characteristic, centrally located, perpendicular spines, etc. 



The true meaning of such facts must yet be discovered, and I 

 know of no more hopeful field of labor for a young naturalist, seek- 

 ing for "new worlds to conquer," than that provided by the fresh- 

 water sponges. The few active workers in this field, in North 

 America, have, thus far, but glanced at a few streams and lakes, 

 mostly in the neighborhoods of Buffalo, Chicago and Philadelphia, 

 and in parts of Florida, Nova Scotia and Newfoundland. There can 

 be little doubt that the rest of the Continent holds many rare prizes 

 in trust for younger and better equipped explorers. 



