July. 1896. CORRESPONDENCE. 71 



narrowness and incompetence. Such men as Cope, Allen, Merriam, Coues, Scudder, 

 Agassiz, Gill, and a score of others in America, and as many other bright lights in 

 England, are systematists. Do let us concede that they are naturalists and 

 scientific men also. 



Lawrence, Kansas, U.S.A. S. W. Willistgn. 



May 24, i8q6. 



[We beg to refer Professor Williston to the Notes on pp. 1-4 of the present 

 number. — Ed. Nat. Sci.] 



What is a " Diagnosis " ? 



Thank you for your note on my paper containing descriptions of new Coccids. 



The only comment it seems worth while to make is in regard to the use of the 

 word Diagnosis. I presume that you will agree that descriptions which accompany 

 names proposed for new species are diagnoses, rather than full descriptions ; that 

 is to say, they aim to present the characters peculiar to the object in question only, 

 or, at any rate, those which distinguish it, taken together, from similar forms. Now 

 my idea of a brief diagnosis, such as those I aimed to give in my paper, is something 

 which will distinguish the species from all species previously known, but which does 

 not necessarily present all the distinguishing characters of the species. A full 

 diagnosis, on the other hand, would give many more characters, with a view to 

 distinguishing the species from others which might be discovered in the future. 

 Even then it is scarcely possible that the list of distinguishing characters would be 

 exhausted ; because, for example, in describing an insect, one never mentions the 

 internal anatomy at all, although if one took the trouble to examine it, I suppose it 

 would present some specific peculiarities. 



Mesilla Park, N.M. Theo. D. A. Cockerell. 



May 13, 1896. 



[As regards the Diagnosis, our view is that of Linnjeus and the masters of 

 systematic description. The diagnosis of a genus gives, as concisely as possible, 

 the characters by which the genus is distinguished from other genera of the same 

 family. The diagnosis of a species gives, equally concisely, the characters by which 

 the species is distinguished from other species of the same genus. Such distinguish- 

 ing points are known as " diagnostic characters." The diagnosis of a family should 

 not repeat ordinal characters, that of a genus should not repeat family characters, 

 that of a species should not repeat generic characters. 



What Mr. Cockerell calls " a full diagnosis," we prefer to call a " Description." 

 This should be kept distinct from the diagnosis, and, without being verbose, should 

 be as clear and complete as possible. Probably it will describe many characters 

 that are not diagnostic, but which either confirm the ascription of the species, genus, 

 or family to its place in the system, or which may prove diagnostic some day, when 

 future species, genera, or families are discovered. As a general rule the discovery 

 of a new species necessitates' the reconstruction of the diagnoses of some, at least, of 

 the species previously known. This is the work that has from time to time to be done 

 by the monographer ; nevertheless, it is incumbent on every describer of a new 

 species to indicate the changes that it renders necessary in our conception of other 

 species. The construction of satisfactory and congruent diagnoses is one of the 

 hardest tasks that a naturalist can set himself. For this reason it is generally 

 shirked by the ordinary species-monger, whose work is on the scientific and literary 

 level of an auctioneer's catalogue. 



As for Mr. Cockerell's final sentence, it is charming in its innocence. Mr. 

 Cockerell, it appears, never mentions, much less examines, the internal anatomy of 

 the numerous species he publishes in all quarters of the world. The sooner he 

 " takes the trouble " to do this the better ; he will find, to take a single instance, that 

 the genitalia of insects afford not the least certain of criteria, as has been amply 

 proved by such eminent workers as Messrs. Salvin and Godman, and as was again 

 insisted on by Messrs. Elwes and Edwards in the revision of the Hesperiidas that 

 they communicated to the Zoological Society of London on June 2. To the mor- 

 phologist we say that external characters have a morphological value ; and to the 



