XYIII PREFACE. 



is for tliis reason, that in several genera, in the choice of which 

 I liave been governed by considerations of expediency, I have 

 enumerated Mr. Walker's species separately, at the end of 

 those genera. 



"What prevented me from carrying out a more complete 

 revision of Mr. Walker's types, was my want of knowledge in 

 many of the families. As I said above, a great deal remains 

 to be done by others. The question has sometimes been raised 

 whether Mr. Walker's descriptions have any claim to priority at 

 all? In my opinion they have, whenever they are recognizable; 

 but they have none, whenever their title to priority can be 

 established only by reference to the type of the description. 

 The characters of some species are so well mai-ked, that a super- 

 ficial description of a single specimen is sufficient for the recog- 

 nition of the species; on the contrary, in other species, some- 

 times in whole genera and families, the specific characters do 

 not lie on the surface, but must be known beforehand or found 

 out. Is such cases Mr. Walker, or any other describer of his 

 type, merely describe the specimen, not the species ; they do not 

 know the species again, when they see it ; consequently, the name 

 they give to that specimen has no scientific meaning at all, 

 and, it seems to me, no claim to priority. A case in point are 

 the North American DoUcliopodldae , described by Mr. Walker. 

 The elaborate and painstaking criticisms of these descriptions by 

 Dr. Loew (Monogr. etc. Vol. II), prove, that Mr. Walker, 

 eitlier from want of knowledge or from carelessness, did not 

 Iiay the slightest attention to those characters which serve to 

 distinguish the species of Dolichopus from each other, so that 

 of th-e twenty -six so-called species, described by him, not a 

 swgle one couM be recognized. Now I ask whether it would 

 le expedient, with Mr. Loew's monograph in hand, to determine 

 Mr. Walker's type-specimens and then to grant to the names, 

 attached to those types, the priority over Mr. Loew's names? 

 I do not tliink so, and, for this reason, I would not undertake 

 that task, even if it were possible*). The same reasoning' 



*) Many of Dr. Walker's species of Dolichopus are represented by 

 female specimens, whicli it would be impossible to determine. 



