328 TAXONOMY. 



same immediate parentage. All plants of the same species are 

 so much alike that they are inferred to have descended from 

 a common stock, and their differences, however grave, are sup- 

 posed to have arisen from subsequent variation, and the more 

 marked differences to have become fixed through heredity. This 

 is included in the idea of species. Descent from a common 

 origin explains the likeness, and is the only explanation of it. 



658. But what is the explanation of the likeness between the 

 species themselves? As respects nearly related species, the answer 

 is clear. Except for practical purposes and in an arbitrary 

 way, no certain and unfailing distinction can be drawn between 

 varieties of the highest grade and species of closest resemblance. 

 It cannot reasonably be doubted that they are of similar origi- 

 nation. Then there are all gradations between very closely and 

 less closety related species of the same genus of plants. 



659. The Theory of Descent, that is, of the diversification of the 

 species of a genus through variation in the lapse of time, affords 

 the only natural explanation of their likeness which has yet been 

 conceived. The alternative supposition, that all the existing 

 species and forms were originally created as they are, and have 

 come down essentially unchanged from the beginning, offers no 

 explanation of the likeness, and even assumes that there is no 

 scientific explanation of it. The hypothesis that the species of 

 a genus have become what they are by diversification through 

 variation is a very old one in botany, and has from time to time 

 been put forward. But until recently it has had little influence 

 upon the science, because no clear idea had been formed of any 

 natural process which might lead to such result. Doubtless, if 

 variation, such as botanists have to recognize within the species, 

 be assumed as equally or even more operative through long ante- 

 rior periods, this would account for the diversification of an 

 original species of a genus into several or many forms as differ- 

 ent as those which we recognize as species. But this would not 

 account for the limitation of species, which is the usual (but 

 not universal) characteristic, and is an essential part of the 

 idea of species. Just this is accounted for by 



660. Natural Selection. This now familiar term, proposed by 

 Charles Darwin, was suggested by the operations of breeders in 

 the development and fixation of races for man's use or fancy ; 

 in animals by breeding from selected parents, and selecting for 

 breeding in each generation those individuals only in which the 

 desired points are apparent and predominant ; in the seed-bed 

 by rigidly destroying all plants which do not show some desirable 

 variation, breeding in and in from these, with strict selection of 



