Dr. W. Marshall on new Stliceous Sponges. 411 
bers will be governed by external influences (quantity of food 
and also again conditions of current). Experimental investiga- 
tions in which sponges, and especially the exceedingly polytropic 
fteniere, are brought under the most various conditions, must, 
I am firmly convinced, lead to the most astonishing results, 
and be of immense importance, not only for Spongiology, but 
also for the whole great theory of transmutation. 
From such considerations it seems not unjustifiable to sup- 
pose that P. Pechuélii and P. Leubnitzie are forms of the same 
species which have become differently developed under differ- 
ent circumstances; but so long as no transitions between 
them are known to us we must regard them as two species. 
It only remains now to explain the reasons which have in- 
duced me to deny gemmules to the Potamolepides. I will lay 
no stress upon the fact that P. Leubnitzie and P. chartaria 
are too thin and have the spicules too closely interwoven to 
give room for such structures, as they might easily have 
acquired other arrangements, such as cavities for their recep- 
tion; but if we consider that, according to what has gone 
before, the formation of gemmules in a tropical freshwater 
sponge is only intelligible during the dry season, while the 
specimens under consideration were, without exception, col- 
lected during the summer months, my view that they form no 
such germ- fragments receives very essential support. The 
specimens are so wonderfully preserved, even to the finest 
marginal parts of the oscula, that there can be no notion of 
their having died the year before or still earlier, and moreover 
it would be singular if the whole number of specimens exa- 
mined were accidentally without gemmules. I have never 
found indigenous freshwater sponges barren (¢. e. without 
products of asexual reproduction) in the autumn and winter, 
and a very considerable number of them have passed through 
my hands in the course of fifteen years. It is indeed 
possible that the non-differentiation of reproductive frag- 
ments might be a consequence of the different conditions of 
existence of the Potamolepides; but this seems to me not very 
probable, and I am more inclined to the opinion that, just 
like the Lubomirskie, and probably also Uruguaya corallordes, 
they have adapted themselves to existence in fresh water at a 
recent date in comparison with the Spongille and Meyenice, 
and that, in case it should become necessary, the formation of 
gemmules may in course of time occur in them also. 
In conclusion, 1 once more beg all my tellow-labourers to 
enable me to extend still further my investigations upon 
freshwater sponges by sending me abundant materials from all 
possible localities. Only the cooperation of many can bring 
30* 
