1919} On Some Generic Distinctions in Sponges 17 



grows the study of a group of species, the more of these mtergrading 

 series come to be known. Hence it may be expected that t^ie char- 

 acters above designated as b and B, will, like a and A be found to 

 intergrade, and c and C likewise, if indeed the recorded data do not 

 already justify that conclusion. 



In view ot the common existence of intergrades between the con- 

 trastin"- members of a pair of generic characters («. and A, e.g.), it 

 becomes in the end impossible to split sponges, organisms in general 

 probably, into genera, each set of which shall represent a particular 

 combination of characters. . 



What practice remains then for the classifier, who knows that with- 

 out systematics, biological data in general cannot be recorded with 

 any certainty that they will be found again, and who therefore must 

 classify, but who does not wish to set up a system of categories which 

 "can be precisely defined only hecause they are arhficml, and into 

 which while some organisms go nicely, others can be brought only 

 after a character is pared down in thought, or extended m thought , 

 One way out, and as I have indicated above, the way into which we 

 have fallen in the case of Tetilla, is to recognize large heterogeneous 

 genera (Tetilla, e.g.) and other smaller, more homogeneous ones 

 ! cannol see at present any better way. It is certainly a preferab e 

 method, i.e., one that enables biologists to find the recorded data no e 

 successfully, than that of building up genera, all of which o^e.lap 



This contrast between small homogeneous genera and large hetero- 

 geneous ones is, like the contrast between homogeneous and hetero- 

 geneous species, perfectly well recognized and widely -^^ m ^*^^^ 

 Ind zoology. Nevertheless the underlying idea is, I believe, not 

 ' always appreciated in its full significance, and in such cases art ifi- 

 clruy defined groups continue to be made, with the single difference 

 that thev are called sub-genera instead of genera. 



Of course, the division of a heterogeneous genus into subgenera is 

 a us ful pra tical method, but if it be assumed that every actual con- 

 crete organism can be put in some one of the neatly defined subgenera, 

 we win be led astray from accurate observation and recording, since 

 In respect to the differential characters intcnncliates exist. 



