STATISTICS OF METASPEKMAE. 615 



nesota valley population with that of 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 Grays 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 Metaspermge to the Metaspermse 

 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, from genera. For the 

 larger regions the larger categories are used as indications of 

 comparative population. So far as concerns the determiua 

 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 



