II. PERMANENCE OF SPECIES. Gl 



inappreciable cliange of the central type, during that 

 short space, to be sufficiently conclusive proof against the 

 gradual transition of species during spaces of time im- 

 measurably more extended. 



" Moreover, we must avoid the straining of our fact be- 

 yond its true bearings. Though the central types of cer- 

 tain species may have remained the same during some 

 scores or centuries of years, this one fact cannot negative 

 a possibility that there are also varieties of the same or 

 of other species which, during the same time, have gra- 

 dually become more and 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. 



" Fui'ther, it is to be kept in memory, that when it be- 

 comes man's interest or pleasure to extend the variations 

 of plants from their central types, he can effect this de- 

 sired result much more rapidly and widely than is seen 

 to occur among 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 dif- 

 ferent 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 

 innuediately through his instrumentality, the changes 



