392 A TEXTBOOK OF BOTANY [Ch. IX 



one based on descent and called phylogenetic, and the other 

 based upon habit and called ecological. 



Phylogenetic classification (the classification par excellence 

 of natural history), expresses the relationships of plants as 

 determined by their evolution or descent from ancestors 

 in common (page 10). It corresponds precisely to blood 

 relationships among men, as expressed in our genealogies, 

 which show who are brothers, who cousins, and who more 

 distantly related. In plants the relationships, of course, show 

 best in those characters which are most constant in heredity ; 

 and these are mostly such, in the higher plants at least, 

 as are connected with the reproductive parts, — whence the 

 emphasis upon flowers and fruits. Thus studied, the Plant 

 Kingdom falls into four primary Divisions, — the Thallo- 

 phytes, Bryophytes, Pteridophytes, and Spermatophytes, 

 which form the subjects of the four following chapters of 

 this book. These Divisions in turn fall into smaller groups 

 called Classes, and they into smaller called Orders, and they 

 into smaller called Families, and they into smaller called 

 Genera, and they into Species, of which some 233,000 are 

 known (page 1). Species are those aggregates of individuals 

 which we recognize as belonging to the same kind, and com- 

 prisable under 'one name. It is true, in some cases they fall 

 into varieties or races, but such are exceptional and their 

 existence does not alter the fact that species are the units in 

 phylogenetic classification. 



The phylogenetic groups are named scientifically, in Latin, 

 upon a somewhat definite plan, though with exceptions due 

 to historical causes. Thus, every species is given a binomial 

 name, composed of the name of its genus, always a noun in 

 the nominative case, in combination with a word associated 

 with that particular species, usually an adjective in agree- 

 ment with the noun. Thus Quercus alba is the botanical 

 name of the White Oak, Quercus being the name of the genus, 

 and alba applied to the species. We have precisely the same 

 usage in English, though in reverse order, when we say White 



