266 PART IV. — VEGETABLE TAXONOMY. 



structure and habits of growth, and which are distinguished from 

 other forms by some constant structural difference. For 

 instance, the Smooth Rose, wherever found, maintains its essen- 

 tial characteristics, and differs constantly in some particulars 

 from the Dog Rose, the Carolina Rose, the Prairie Rose, etc. 

 Such a form is typified by one of the twigs of our figurative tree, 

 and we call it a species. Species often resemble each other, as 

 do the different species of Roses just mentioned. Such a group 

 of species we call a genus, and that to which the roses belong we 

 name the genus Rosa. Just as species naturally fall into genera, 

 so these, in turn, form higher groups, called orders. The genus 

 Pyrus, for example, which contains the Apple, Pear and Quince, 

 resembles that of Rosa in certain important and constant par- 

 ticulars in which it differs from all other plants. The same is 

 true of the genus Rubus, which contains the Raspberries and 

 Blackberries, the genus Crataegus, which contains the Hawthorns, 

 and so on. These genera, therefore, are placed together in the 

 order Rosacea. Similarly, orders which resemble each other 

 form classes ; classes which resemble each other are called series 

 or branches, and these are the primary divisions of the plant 

 sub-kingdom. 



It must not be understood that groups of the same name are 

 always equal either in the sense of being equally numerous in 

 sub- divisions or individuals, or in the sense of being marked off 

 with equal distinctness from other groups ; on the contrary, they 

 are often very unequal in both senses, as we should naturally 

 expect if we bear in mind our figure of the tree. Some species 

 for example, are rare, they contain but few individuals, and these 

 may not fall into distinct sub-groups or varieties ; while others 

 are exceedingly numerous, and may be broken up into many 

 varieties ; some species, at least so far as existing forms are 

 concerned, are sharply marked off from other species, while 

 others shade so insensibly into other species, by reason of con- 

 necting forms, that it is often difficult to draw the line between 

 them. So it is also with the higher groups, genera, orders 

 and classes. When genera are large, they are often conve- 

 niently divided into sub-genera; large orders are commonly divi- 

 ded into sub-orders, and these, perhaps, again into families and 

 tribes. Classes, also, are often divided into sub-classes. The 

 terms race and variety are applied to subdivisions of species. 



