12 THE SPONGES. 



they were rampant, to-day they are reduced to a minimum, with the 

 result, as I have said, that the species of modern sponge literature are strik- 

 ingly homogeneous groups, which need not be thought of as always corre- 

 sponding to natural races. 



That this method of precise analysis is the only method capable of 

 yielding trustworthy data, seems to me incontestable. 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 belonging to the 

 same race. Such knowledge, to be acquired through continuous observa- 

 tion of living individual sponges under normal and under modified condi- 

 tions (experimental method) may be expected to bring about the imion of 



many recorded species. Another most important class of data can only 



be revealed through the physiological study of the race, viz. through the 

 breeding of sponges. And with the increase in the number of marine 

 laboratories at which observations may be carried on continuously through- 

 out the year, the inauguration of such studies may be anticipated. The 



modern statistical method of considering the differences between indi- 

 viduals of such groups as are procurable in large numbers is a refinement 

 of what is commonly understood as systematic work, and a promising field 

 for those acquainted with the structure of sponges. Such studies, by 

 revealing the kinds and the extent of structural modifications which occur 

 among individuals not separable into morphologically definable groups, may 



be expected to provide invaluable special cases for experimental study. 



It io through the combination of these several classes of data that we must 

 hope to learn the limits of the natural groups of sponges as they exist 

 to-day. When such trustworthy definitions of natural groups are at hand, 

 the facts of the geographical distribution of the species will doubtless 

 become intelligible. 



