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become more aud more unlike their respective central types ; until, 

 through diminished likeness, they may now actually be referred to 

 different central types, — that is, may be described as distinct species. 



Further, it is to be kept in memory, that when it becomes man's 

 interest or pleasure to extend the variations of plants from their cen- 

 tral types, he can effect this desired result much more rapidly and 

 widely than is seen to occur amongst plants in a state of nature. By 

 taking the more decided varieties as parents of a fresh stock, through 

 several successive generations, and so gradually rendering them more 

 and more unlike the central type, we appear to weaken their tendency 

 to resume that type. Hitherto, no limit has been ascertained to this 

 power of changing plants by varying varieties. Some of the species 

 which have been long subjected to this process, have been run into 

 varieties so widely different from their known or supposed central 

 types, that if any botanist had first found their extreme forms in a 

 newly explored country, he would assuredly have believed them to 

 belong to different central types — to be totally distinct species. 



In this, as in every other such process, man works only with the 

 powers of nature. Although brought about immediately through his 

 instrumentality, the changes effected in the plants are simply the na- 

 tural results of those conditions to which he subjects them. There 

 seems no reason to suppose that the same result which man brings 

 about more rapidly, could not have been brought about, though more 

 gradually, without his interference. If man can produce hereditary 

 varieties of plants, which remain permanently different from their cen- 

 tral types, under his care, why cannot nature also produce such heredi- 

 tary varieties ? And in what respect does an hereditary variety, the 

 origin of which is unknown, differ from a species ? 



Looking to present events in nature, and to results produced by the 

 interfering agency of man, the following conclusions seem reason- 

 able : — 



1st. — The central type of a species is reproduced and remains the 

 same through many successive generations. 



2ndly. — Nevertheless, individual plants do occasionally differ more 

 or less widely from their central type, and thus become varieties. 



3rdly. — The descendants of varieties frequently revert to the central 

 type of the species from which those varieties originated. But we 

 cannot show that all varieties eventually do thus revert. 



4thly. — The effects of cultivation, in rendering varieties more differ- 

 ent, and perhaps more permanently different, from their central types, 

 together with the occurrence of hereditary varieties among wild plants, 



