igof\ Wilson — Sponges. 43 



cus, Littorina litorca, or other organisms which we know in 

 great number, is not only open to doubt, but is excessively 

 improbable. It is, I suppose, from this latter point of view 

 (the envisaging clearly the a priori probability that sponges 

 in general exhibit those individual and local differences which 

 all species known intimately exhibit) that O. Schmidt was led 

 to record in literature the existence of such species as Farrea 

 facunda. Perhaps Farrea facunda is a "natural species," but 

 the data at hand make such a statement only a subjective 

 assumption. Or when the distinguished systeraatist Topsent 

 expresses the opinion that five species of Poecillastra 

 recorded by Sollas probably represent the variations of two or 

 three species, one is justified in saying "perhaps, but the 

 known specimens differ in certain definite respects." Such 

 subjective interpretations of differences perhaps always affect 

 the manner in which we record the occurrence of certain mor- 

 phological peculiarities in association with geographical and 

 bathymetrical site. But whereas once they were rampant, 

 today they are reduced to a minimum, with the result, as I 

 have said, that the species of modern sponge literature 

 are strikingly homogeneous groups, which need not be 

 thought of as always corresponding to natural races. 



That this method of precise analysis is the only method 

 capable of yielding trustworthy data, seems to me incontest- 

 able. That it may result in temporarily recording more 

 species than exist in nature, will only trouble those who 

 incline to the view that the one excuse for systematic zoology 

 is to provide them with a handy collection of names for the 

 animal kingdom. 



The data which are thus accumulating as to the occurrence 

 of this or that peculiarity of structure in a certain locality 

 are growing rapidly through the labors of systematists. 

 Scarcely begun is the accumulation of the almost equally 

 important data (comp. Polejaeff, Report on the "Challenger 

 Keratosa," p. 85), as to what peculiarities of structure are 

 due to a difference in the physiological state of individuals 



