PREFACE. 



This series of monographs was undertaken, 

 not only to give an account of the species, but 

 also of the varieties, and, as far as the material 

 at hand would permit, to indicate the extent 

 of variation and its possible limit, if there is 

 such a limit in nature. We do not, in fact, 

 know a species properly until we are acquainted 

 with the different appearances it assumes, and 

 the approaches it makes to other species. 

 Some species require years of study from ex- 

 tensive series, with an examination of the 

 entire animal, and observation of their habits 

 in their native localities ; but under the most 

 favorable circumstances doubtful points will 

 remain, because many species are less distinct 

 in nature than they are represented to be in 

 books. 



In the plates, the longitudinal lines made 



