These are the main and immediate causes of the great strides that the knowledge of the natural productions of North America has made within a period of little upwards of twenty years. There are, of course, others, of which the general diffusion of knowledge and attention to education in the United States especially, and in fact throughout the civilized world, have been perhaps the most efficient. Since the time of the publication of the works of our predecessors in American Ornithology, the additions of species and of information in that department of our Zoology have been very large, and being for much the greater part within our reach, we have been induced to undertake the present work. It is to be regarded in some measure as an addition to the works of former authors in American Ornithology, but at the same time complete in itself. Our advantages for study have been much superior to those possessed by former writers in America. There never was in the United States, until within the last ten years, a library of Natural History, approximating in any considerable degree to completeness, nor affording the necessary facilities for the study of Ornithology. Nor until within that period was there any collection sufficiently comprehensive to answer the purposes of comparison and general research. In various branches of Natural History, but especially in Ornithology, these most important and desirable objects have been fully accomplished in the formation of the Library and Museum of the Academy of Natural Sciences of Philadelphia,—a result for which science is indebted, not to the Government, nor to public patronage of any kind, but to strictly private enterprise and individual scientific taste. A new era in the history of the zoological sciences in the United States commenced with the purchase and importation of the late General Massena’s collection of Birds into this country in 1846, by Thomas B. Wilson, M. D., of Philadelphia, and the commencement at the same period of a library in Natural History by the same gentleman and his brother, Mr. Edward Wilson, now residing in England. Both have been continued without intermission to the present time, and the results have been, so far as relates to Ornithology, that a most extensive collection has been formed, now numbering about twenty-five thousand specimens, and a Library containing very nearly every book relating to this branch of natural science, of which copies are to be obtained; and also, principally through the personal exertions of the latter-named gentleman, in the cities of Europe, many that are of exceeding rarity and value. In several other departments, also, nearly or quite as complete collections of books and specimens have been made. To Fossil Remains, Conchology, and Crustacea, these remarks are especially applicable.