4 MANUAL OF BOTANY 



generations — but they gradually revert to one or the other 

 parent stock. 



We have now seen that species, under certain circumstances, 

 are liable to variations, but that all such varieties have a ten- 

 dency to revert to their original specific type. Hence, from a 

 practical point of view, species must be considered as permanent 

 productions of Nature, which are capable of varying within 

 certain limits, but in no cases capable of being altered so as to 

 assume the characters of another species. There is not the 

 slightest foundation for the theory, which has been advocated 

 by some naturalists, of a transmutation of species. All such 

 statements, therefore, that have been made, of the conversion 

 of Oats into Rye, or of any species whatever into another, are 

 entirely without foundation, and have arisen from imperfect 

 observation. 



In practice it is important that we should distinguish 

 varieties from true species, for nothing is so calculated to lead 

 to confusion in Descriptive Botany as the raising of mere 

 varieties to the condition of species. No individuals should be 

 considered as constituting a species unless they exhibit important 

 and permanent distinctive characters in a wild state. Great 

 uncertainty still prevails in our systematic works as to what is a 

 species and what is a variety ; and hence we find different 

 authors, who have written on British and other plants, estimate 

 the number of species contained in such genera as Bosa, Buhus, 

 Saxifraga, Hieracium, Salix, Smilax, and others, very 

 differently. 



2. Geneea. — The most superficial observer of plants will 

 have noticed that certain species are more nearly allied to each 

 other than to other species. Thus, the different kinds of Eoses. 

 Brambles, Heaths, Willows, may be cited as familiar examples 

 such assemblages of species ; for, although the plants com- 

 prehended under these names present certain well-marked dis- 

 tiQctive characters, yet there are at the same time also striking 

 resemblances between them. Such assemblages of species are < 

 called genera. A genus, therefore, is a collection of species 

 which resemble each other in general structiu-e and appearance 

 more than they resemble any other species. Thus, the various 

 kinds of Brambles constitute one genus, the Roses another, the 

 WUlowB, Heaths, Clovers, and Oaks form also, in like manner, 

 as many different genera. The characters of a genus are taken 

 exclusively from the organs of reproduction, while those of a 

 species are derived generally from all parts of the plant ; hence 



