I4 NORTH AMERICAN DIPTERA. 



The answer to the second query, What is a genus? is, 

 however, a very different matter. Ordinarily we might 

 apply the same criterion, that groups of species gradu- 

 ally blending together should not be separated into two 

 or more genera. But this will not suffice, since, because 

 of the actual presence of the connecting links, the ex- 

 tremes may vary enormously, far more than in many cases 

 where the connecting links have disappeared, leaving 

 the extremes isolated into easily distinguishable genera. 

 Both convenience and the demands of relationships re- 

 quire here that such groups be broken up, though it may 

 and often does entail the result that such genera may be 

 ultimately distinguishable in their most allied species by 

 only trivial characters. But the temptation offered here, 

 especially to the narrow, perspectiveless specialist, is to 

 use those same boundary characters, or their equivalents, 

 as generic characters through the whole family, and the 

 result is an almost innumerable number of proposed di- 

 visions. As nearly every species of flies has some plastic 

 or structural distinguishing character, it is very evident 

 that we might ultimately reach the absurd result of mak- 

 ing species and genera coterminous. Between this ex- 

 treme and the other, the grouping of large numbers of 

 species into genera, all of which can be distinguished by 

 decisive, if not important, structural characteristics, there 

 must be a happy mean. This mean, however, must de- 

 pend more or less upon the opinions of those best quali- 

 fied to interpret them. In other words I am tempted to 

 define a genus as being merely the personal opinion of its 

 proposer. By an excessive 'splitting' of genera, broader 

 relationships are lost sight of, and the tendency is inevit- 

 able to restore those evidences by the invention of new 

 group terms to express them. Perhaps no better exam- 

 ples of these tendencies are observable than in the more 

 recently proposed classification of the mosquitoes. For 



