List of the Natural Orders of the Flora of the Northern United States, with the number of Genera and Species comprised in them,-distinguishing the introduced and the indigenous Species,-and of the indigenous Species common to this district and to Europe.

CLASS I. DICOTYLEDONE S. EXOGENÆ.

It is plain enough that the numbers in this tabular view must be essentially influenced throughout by one's views as to the limitation of species and genera. In the hands of a few botanists, the flora of the Northern States might exhibit a somewhat smaller number of species than it here does; but with most, there would undoubtedly be a stronger tendency in the opposite direction. As it is obviously impossible at present to reduce the various ideas and shades of difference that prevail respecting species to one common standard, all that can be done is to indicate the bias, or what astronomers call the personal equation, of each author, which must be duly considered when different

floras are to be compared. This is not the place to discuss the principles involved in the general question, nor to explain or defend any conclusions to which I may have arrived;-except to say that my determination of species in each particular case has been based on the evidence before me as irrespective of all theoretical considerations as possibly could be. It is necessary to state, however, that, so far as I can judge, the authors of the principal and most esteemed recent European Floras, if in my place, would be likely to increase the present number of our Phænogamous plants and Ferns about five per cent. One school, indeed, would doubtless add at least ten or twelve per cent. to the species here received, and give results quite incommeasurable with my own. I can only say, on my own part, that an enlarged experience certainly inclines one to take broader views of species than those which prevail among the generality of European botanists.

The numerical comparison of our Phænogamous with our Cryptogamous species, however interesting it might become in a complete flora, is here of little moment; only the higher Cryptogamia being included. Moreover, it should be noted that the Musci and Hepatica enumerated in the above table are those of a geographical area about twice that of the higher or Acrogenous Cryptogamia and the Phænogamia. For the distinguished American muscologist who elaborated these two orders for our 'Botany of the Northern States,' anxious to afford facilities for the study of our mosses throughout the country, has included all known to him within the whole United States east of the Mississippi, and even some as yet found only to the north and west of these limits. It is evident, also, that the number of forms admitted as species is proportionally larger in these two orders than in the rest of the work. On the other hand it is to be considered how little our mosses have as yet been collected and studied, and how likely it is, in view of their general wide range, that most of these outlying species may yet be detected within the Northern States, including Virginia and Kentucky.

We naturally restrict our attention mainly to the Phænogamous vegetation, as best known in all countries and affording the most precise data for comparison. And we exclude at once the 260 introduced species, most if not all of which have become denizens of our country since its settlement by Europeans, and in consequence of that settlement;-leaving the question of their origin, introduction, &c., for future consideration. Their admission into the account in the comparing our flora with that of Europe, as has been done, seriously vitiates our conclusions.*

* Thus Mr. Watson, as cited by Alph. DeCandolle (Geogr. Bot. p. 511) enumerates 602, out of 1428 phænogamous British plants, as common to Great Britain and America. I count only 321 out of 2091 phænogamous species indigenous to the Northern United States as indigenous also to Europe.