INTRODUCTION. XV 



will not be surprised, therefore, at my not repeating, with respect 

 to genera, the characteristics peculiar to every species or to the 

 greatest number of them. In short, there are still other charac- 

 teristics which I pass over, considering them rather useless either 

 on account of their constancy (such as the presence of silky hairs 

 on the tibia?) or on account of their variableness, such as the color 

 of the lower surface of the abdomen. 



Descriptions are often made prolix by means of these super- 

 fluous indications, and thus the essential characteristics are 

 drowned in useless developments. In this way, precision is 

 diminished instead of being increased. Doubtless, here again 

 nothing is absolute. Certain isolated species may be sufficiently 

 characterized by some salient traits, while others surrounded by 

 very closely connected species, require minute descriptions. 1 The 

 first condition of good comparative diagnoses resides in a wise co- 

 ordination of the species which by way of exclusion may lead to 

 choosing only between a small number of species. Though I do 

 not like to find fault, I cannot, however, on this score, help com- 

 plaining of the works in which the species, though described in 

 an absolute manner (that is, by themselves and not comparatively 

 with others) are jumbled up together without order, without 

 division of genera, often in defiance of the most salient character- 

 istics. 



Such works, got up in a hurry, the plans of which are laid 

 down with a view to the convenience of the authors and not for 

 that of the readers, cause the latter to lose much valuable time 

 with no great result. They do not come up to the precision now 

 required by the progress of science, and they are therefore behind 

 their time. The reader cannot occupy his mind with incomplete 

 works, nor can he waste his time in striving to find out species 

 which are not to be found out ; for there is no doing impossibilities. 



In most of my descriptions, I have been especially attentive to 

 the forms and characteristics of the forms and carving, attributing 



1 Absolute and very detailed descriptions ought, in ray opinion, to be 

 employed, when one describes species isolatedly, without knowing the 

 most closely connected types (for instance, in the publication of geogra- 

 phical expeditions). It is the monographer's duty to eliminate, from these 

 descriptions, both the commonplace and the useless. But in a mono- 

 graph, the species are to be examined in a comparative manner, and rela- 

 tively to the adjacent types. 



