Natural History of the United States. 259 



is frequent, others never change, some change periodically, others 

 accidentally ; some throw off certain ornamental appendages at 

 regular times, the Deers their horns, some Birds the ornamental 

 plumage they wear in the breeding season, etc. All this should 

 be ascertained for each, and no species can be considered as well 

 defined and satisfactorily characterised, the whole history of 

 which is not completed to the extent alluded to above. The 

 practice prevailing since Linnseus of limiting the characteristics 

 of species to mere diagnoses, had led to the present confusion, of 

 our nomenclature, and made it often impossible to ascertain what 

 were the species the authors of such condensed descriptions had 

 before them. But for the tradition which has transmitted, gene- 

 ation after generation, the knowledge of these species among the 

 cultivators of science in Europe, this confusion would be still 

 greater ; but for the preservation of most original collections it 

 would be inextricable. In countries, which, like America, do not 

 enjoy these advantages, it is often hopeless to attempt critical in- 

 vestigations upon doubtful eases of this kind. One of our ablest 

 and most critical investigators, the lamented Dr. Harris, has very 

 forcibly set forth the difficulties under which American natural- 

 ists labor in this respect, in the Preface to his Report upon the 

 Insects Injurious to Vegetation." 



We have been led by the great interest of the subject into so 

 long a discussion of the points already referred to, that it will be 

 impossible to notice many others equally important, as for instance 

 the application of the general views above discussed ; or to say 

 anything of the more special subject of the volume, the Embry- 

 ology of the American Tortoises, so ably described and beautifully 

 illustrated. Nor will it be possible to enter on the views given of 

 the relation of embryonic development to classification and geolo- 

 gical sequence, — a most tempting subject, though at present en- 

 compassed with a crowd of difficulties and apparent exceptions 

 that await for their solution and explanation such investigations 

 as those which now occupy Agassiz. 



In conclusion, every true naturalist will endeavour not only to 

 read but carefully to study this work, the high merits of which 

 we do not wish to depreciate, however we may be constrained to 

 differ from some of its more general doctrines. Ao-assiz himself 

 will be the last to require an implicit assent to his views, merely 

 because he holds them ; and we know that he values truth too 

 much, and is too deeply imbued with reference for nature and its 



