SYSTEMATIC BOTANY 287 



garded, they are considered to form one species, just as all 

 the common morning-glories, of whatever shade or color, 

 belong to the species Ipomea purpurea. The small differ- 

 ences that arise within a species as to the color and size of 

 flowers, and other minor points, constitute mere varieties 

 and have no special names applied to them. The line 

 between varieties and species is not clearly defined, and in 

 the nature of things can never be, since progressive devel- 

 opment, through slow but unceasing change, is the law of 

 all life. 



In botanical descriptions the name both of the species 

 and of the genus is given, just as in designating a person, 

 like Mary Jones or John Robinson, we give both the 

 surname and the Christian name. The genus, or generic 

 name, answers to the surname, and that of the species to 

 the Christian name except that in botanical nomencla- 

 ture the order is reversed, the generic, or surname coming 

 first, and the specific or individual name last ; for example, 

 Ipomea is the generic, or surname, of the morning-glories, 

 and purpurea the specific one. 



412. How to use the Key. Any good manual will do ; 

 Gray's " School and Field Book " is perhaps the best avail- 

 able at present for the States east of the Mississippi. A 

 little reference to what has already been said on the subject 

 of classification in Sections 126-129, will make its use 

 clear. Suppose we want to find out to what botanical 

 species the morning-glory, or the sweet potato, for instance, 

 belongs. Turning to the key we find the sub-kingdom 

 of Phaenerogams flowering, or seed-bearing plants 

 divided into two great classes, Angiosperms and Gymno- 

 sperms, as already explained in the Sections referred to. 

 A glance will show that our specimen belongs to the 

 former class. Angiosperms, again, are divided into the 

 two subclasses of Dicotyledons and Monocotyledons 

 (Sec. 129). We at once recognize our plant, by its net- 

 veined leaves and pentamerous flowers as a dicotyledon 

 (Sees. 37, 302), and turning again to the key, we find this 



