352 CLASSIFICATION. 



version to the primitive type, and so secures his particular end. 

 The races of Corn, Wheat> &c., which now preserve their charac- 

 ter unchanged, have become fixed by centuries of domestication. 

 Even these, at times, manifest an unequivocal disposition to return 

 to their aboriginal state. Were cultivation to cease, they would all 

 speedily disappear ; the greater part, perhaps, would perish out- 

 right ; the remainder would revert, in a few generations of sponta- 

 neous growth, to the character of the primitive stock. 



671. Hybrids OF Cross-breeds, Variations of a still different class 

 are artificially, and sometimes spontaneously, produced, by fertiliz- 

 ing the ovary of one plant with the pollen of a nearly allied spe- 

 cies ; from which arise what are called Cross-breeds , or Hybrids. 

 Crosses between different species, however, are almost always in- 

 capable of producing fertile seed, and therefore are not perpetu- 

 ated in nature : those between distinct varieties of the same spe- 

 cies are usually fertile, and give rise to new sets of varieties (also 

 termed Races), in which the particular qualities of their immediate 

 parents are variously modified or blended ; but which, by a contin- 

 uation of the same influences, revert to one or the other parent 

 stock. 



672. Genera, If but a moderate number of species were known, 

 no system of generalizing, or arranging them in groups, would be 

 necessary for ordinary purposes ; though a consideration of the 

 various degrees of resemblance between different species could not 

 fail to suggest some form of generalization, like that which the 

 great number of species early rendered necessary. The first step 

 in proper classification, the bringing together of species into kinds, 

 according as they are seen to resemble each other, is almost as 

 natural and inevitable an operation of the mind, as is the idea of 

 species involuntarily deduced from the assemblage of like individ- 

 uals. The generic association, however, implies only resemblance, 

 or similarity of kind, not identity of origin. A GENUS, therefore, 

 is an assemblage of nearly related species, formed after the same 

 pattern, and therefore agreeing with one another in general struc- 

 ture and appearance. Thus, the wild Swamp Rose, the Sweet- 

 brier, the Dog Rose, French Rose, Cinnamon Rose, and others, 

 constitute the universally recognized genus Rosa ; the various spe- 

 cies of Raspberry and Blackberry compose the genus Rubus ; the 

 Apple, Pear, &c., form the genus called by botanists Pyrus : so 

 the different Oaks, Willows, Poplars, Birches, &c., form as many 



