Ylll PREFACE. 



all the above-mentioned persons for the assistance which they 

 have kindly afforded me. Among them I ought to name espec- 

 ially, Prof. Henry, as director of the Smithsonian Institution, and 

 Prof. Baird, who have always shown me the greatest kindness. 



I ought, however, to inform the reader that, notwithstanding so 

 much help, the work does not contain all the species of America. 

 I myself possess divers individuals, which, without offering 

 characteristics distinct enough to be described from one indi- 

 vidual, nevertheless betray the existence of species yet unknown. 

 Moreover, the larger part of the smaller species seem till now to 

 have baffled the researches of collectors. I do not hesitate to 

 affirm, that, had I been able to collect undisturbedly in the United 

 States, I should very likely have brought back from that country 

 a number of small species as considerable as that which I have 

 brought from Mexico. 



But it is in the natural progress of science to advance gradu- 

 ally towards completeness ; and consequently, it is the lot of 

 books of science to grow old and become obsolete ; and thus, in 

 their turn, to give way to more complete works. I do not, there- 

 fore, consider this monograph, incomplete as it may be, without 

 its use. I am not one of those who suppose it possible to 

 exhaust a subject of investigation. On the contrary, I am of 

 opinion that in entomology, as well as in the other branches of 

 science, nothing perfect, nothing absolutely complete, can be 

 accomplished, seeing that everything in nature is undefined. The 

 naturalists who think the contrary, and who are induced constantly 

 to put off the publication of their labors from believing they 

 shall wholly complete them, succeed but too often in losing the 

 fruits of their studies ; either because their writings grow old 

 while lying by, or because the authors are overtaken by death. 

 The proper method in the study of nature is not that of per- 

 fection, but, on the contrary, that of approximation. Because 

 from approximation to approximation, we are always getting 

 nearer exactness and completeness, without ever attaining extreme 

 perfection. In that precisely consists the progress of science, and 

 that also it is which gives to its horizon the depth of infinitude. 



