KEPEODUCTrVE OEGANS OF PLANTS. 297 



Species, therefore, cannot change any of their essential 

 characters, those characters which are hence termed specific. 



TarietieSi — Individuals of the same species differ. In 

 fact, no two individuals are quite alike. Circumstances of 

 temperature, food, and hahits of life, increase these differ- 

 ences, and varieties originate when such differences assume 

 a comparative permanence and fixity. But as external 

 conditions cause variation away from any particular rep- 

 resentative of a species, so they may cause variation back 

 again to the original, and although variation may take a 

 seemingly wide range, its hounds are fixed and do not 

 touch specific characters. 



The causes that produce varieties are numerous, bu.t in 

 many cases their nature and their mode of action is diffi- 

 cult or impossible to understand. The influence of scarcity 

 or abundance of nutriment we can easily comprehend may 

 dwarf a plant or lead to the production of a giant indi- 

 vidual ; but how, in some cases, the peculiarities thus im- 

 pressed upon individuals acquire permanence and are 

 transmitted to subsequent generations, while in others 

 they disappear, is beyond explanation. 



Among plants, varieties may often be perpetuated by 

 the seed. This is true of our cereal and leguminous 

 plants, which reproduce their kind with striking regulari- 

 ty. Other plants cannot be or are not reproduced unalter- 

 ed by the seed, but are continued in the possession of their 

 peculiarities by cuttings, layers, and grafts. Here the in- 

 dividual plant is in a sense divided and multiplied. The 

 species is propagated, but not reproduced. The fact that 

 the seeds of a potato, a grape, an apple, or pear, cannot be 

 depended iipon to reproduce the variety, may perhaps be 

 more commonly due to unavoidable contact of pollen 

 from other varieties, than to inability of the mother plant 

 to perpetuate its peculiarities. That such inability often 

 exists, is, however, well established, and is, in general, 

 most obvious in case of varieties tliat have to the greatest 

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