44 Journal of the Mitchell Society. [March 



belonging- to the same race. Such knowledge, to be acquired 

 through continuous observation of living individual sponges 

 under normal and under modified conditions (experimental 

 methods may be expected to bring about the union of many- 

 recorded species. Another most important class of data can 



only be revealed through the physiological study of the race, 

 viz. through the breediug of sponges. And with the increase 

 in the number of marine laboratories at which observations 

 may be curried on continuously throughout the year, 

 the inauguration of such studies may be anticipated. 

 The modern statistical method of considering the differ- 

 ences between individuals of such groups as are procurable in 

 large numbers is a refinement of what is commonly under- 

 stood 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 morpholog- 

 ically definable groups, may be expected to provide invaluable 



special cases for experimental study. It is through the 



combination of these several classes of data that we must 

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

 as they exist today. When such trustworthy definitions of 

 natural groups are at hand, the facts of the geographical dis- 

 tribution of the species will doubtless become intelligible. 



