78 Statistics of the Flora of the Northern States. 
genera. Of these only 18, or 10°5 per cent, are monotypic, while 
13°3 per cent of our whole number of genera are mo 
23 genera or almost 18 per cent contain only two or at onal 
good ies apiece, and about as many more have only four or 
five good species. ‘Therefore 64 of the smaller genera, or 37 per 
cent, fully come up to the —_ average of species to genus in 
our ’Phenogamous plants generally, viz. three each (supra, p. 
216); while on the other sco, 15, or 8 per cent, are very large 
genera, such as Carex, So idago, yperus, Saliz, A llium, Dalian 
Trifolium, Gentiana, Ranunculus, &e. Though | many of these are 
not very large genera in our region, nor do those that are large 
particularly abound in amphigzan species 
Comparison of the Flora of the eth United States with that 
of Hurope in respect to the Similar or Related Species. 
Two floras may be, perhaps, as nearly related through their 
allied as through = onan species: at any rate, the compari- 
son in this respect is oer important to be made. Such com- 
parisons, however, are vitae more difficult, owing to the impos- 
sibility of estimating the degrees of resemblance among species, 
or at least of a5 nom them in any precise or definite way, or 
of bringing shades of ps a to any common standard. In 
ry, indeed, es one grade of resemblance is su to 
expressed in genera, But genera,—even those whose circum- 
scription is either dléaély defined in nature (which is far from 
being always the case) or is generally agreed upon, are by no 
means groups of equal value throughout; and the species of 
every genus, when “ion or numerous, resemble each other in 
very unequal degree 
Still no two enatayth but geographically separated floras of 
any size are so well known, as to their Phzenogamia, and afford 
generally such facilities for the comparison of their at spe- 
cies, as those of the Northern United States and of Northern 
Euro 
if we » judge of their relationship from the large proportion of 
the genera common to the two, we might infer it to be very close. 
article, o 2 216 et seg., we count 326, or not much less than 
onteball of the 681 genera as belonging also to Europe. This 
a great amount of related vegetation in the two floras, 
#0 doubt; but of the degree of relationship, taken comparatively, 
it gives us no correct idea, until we know how many of the gen- 
era common to the two are almost cosmopolite, or are wide-spread 
over the cooler parts of nearly the whole northern hemisphere; 
and how many are peculiar or strictly characteristic. Now, on 
going over the list, I find that an extraordinarily large proportion 
of the genera common to our flora and to Europe belong also 
