HISTORY OF LITERATURE. 11 



must be treated as one ; the ' Deutschlands Crusta- 

 ceen,' etc., being wholly descriptions and figures of 

 species ; the ' Uebersicht,' although a separate work, is 

 really the classificatory part of the other work, which 

 was published in fasciculi of small loose leaves without 

 any arrangement whatever. In c British Oribatidae,' 

 page 21, I have already treated fully of the nature of 

 Koch's works, therefore I shall do so very shortly here ; 

 they were general treatises, from a systematist's point 

 of view, upon the whole of the Acarina. Each creature 

 that Koch considered a species was described and 

 figured in colour. Koch was an admirable collector, 

 and a perfectly honest naturalist; he doubtless saw 

 everything he figured, but the task was too vast for 

 accuracy. The smallest difference of sex, age, or colour, 

 even though given by the food last consumed, was 

 sufficient to found a species upon. When it comes to 

 classification most of his genera have stood by virtue 

 of what later writers have selected as type species ; but 

 Koch had not a genius for classification, and his defini- 

 tions of genera usually fail to convey any distinct ideas 

 to the reader's mind. Except in unusually well-marked 

 species identification by Koch's book is very difficult. 

 Koch apparently paid less attention to the Tyroglyphidge 

 than to any other family of Acari which he knew of. 

 He included all in the genus Acarus except two, which 

 were hypopial nymphs. One of these he at first 

 erroneously called a Dermal eichm, but in the ' Ueber- 

 sicht ' he included both in Homopus. It is now known 

 that a Homopus is only one sort of hypopial nymph. 

 Curiously enough Koch placed the so-called genera 

 Hypopus and Homopus rather far apart ; he placed his 

 genus Acarus among the Sarcoptida3, but said he thought 

 it could not remain there permanently. Of the ten 

 species remaining in Koch's work after removing the 



Beitrag zur deutschen fauna," in 40 Hefts, Regensburg, 1835-44. 

 Each heft was published simultaneously as one heft of Herrich- 

 Schaffer's edition of Dr. G. W. F. Panzer's 'Deutschland's Insecten. 

 Uebersicht der Arachnidensystems,' Niirnberg, 1837 1850 (in Hefte). 



