STATISTICS OF METASPERMAE. 615 
nesota valley population with thatof such a commonwealth as 
Nebraska, of which careful floras have been compiled, has 
already been intimated in the introductory chapter of this 
work. A political district can not have any distinct meaning 
in a plant-distributional inquiry. So, too, a comparison be- 
tween the species of the Minnesota valley and those of the 
Atlantic United States, as compiled in Watson and Coulter’s 
edition of the Gray’s Manual, or between the valley species and 
those of the southern states, as compiled in Chapman’s Flora 
of the Southern States, would be of doubtful value and nothing of 
the sort has been attempted. The idea has been, as stated, to 
analyse the plant-population with a view of discovering the 
preponderance-ratios of various distributional and physio- 
gnomic elements. 
Points of statistical investigation. In a relatively circum- 
scribed area, specific forms—and with these I. have always 
included varietal forms as of the same implication—are more 
valuable than generic, and generic characters more important 
than family or ordinal characters. Being more limited and 
more definite, they are at the same time more easily handled 
with approximate exactness and more instructive than charac- 
ters of a greater generality. The principal compilations for 
the North American continent comparisons are of specific 
ranges and characteristics. But in determining the relation- 
ship of the Minnesota valley Metaspermee to the Metaspermee 
of the whole northern hemisphere, and of the world, the gen- 
eric or family characters come into play as the more convenient 
and more exact for purposes of comparison. The general po- 
sition of the Minnesota valley in the plant-population regions 
of the earth is first examined from the statistics of families. 
Next, the position of the Minnesota valley as an area of the 
northern hemisphere is determined, principally from the sta- 
tistics of genera, although to some extent, also, from species. 
Last, the position of the Minnesota valley in the North Amer- 
ican continent is determined principally from the statistics of 
species, although to some extent also, frum genera. For the 
larger regions the larger categories are used as indications of 
comparative population. So far as concerns the determina- 
tion of physiognomic characters only specific forms have been 
tabulated, for it is to species and not to genera that the plant- 
physiognomy of any region is to be referred 
