12 ORIGIN OF CULTIVATED PLANTS. 



sphere a phenomenon of several thousand years' duration, 

 if we are to judge from the size of the deposits transported 

 by the ice ; but cultivation began after this epoch, and 

 even in many instances within historic time. We have 

 little to do with previous events. Cultivated species 

 may have changed their abode before cultivation, or in 

 the course of a longer time they may have changed their 

 form ; this belongs to the general study of all organized 

 life, and we are concerned only with the examination 

 of each species since its cultivation or in the time 

 immediately before it. This is a great simplification. 



The question of age, thus limited, may be approached 

 by means of historical or other records, of which I shall 

 presently speak, and by the principles of geographical 

 botany. 



I shall briefly enumerate these, in order to show 

 in what manner they can aid in the discovery of the 

 geographical origin of a given plant. 



As a rule, the abode of each species is constant, or 

 nearly constant. It is, however, sometimes disconnected ; 

 that is to say, that the individuals of which it is corn- 

 posed are found in widely separated regions. These cases, 

 which are extremely interesting in the study of the 

 vegetable kingdom and of the surface of the globe, are 

 far from forming the majority. Therefore, when a culti- 

 vated species is found wild, frequently in Europe, more 

 rarely in the United States, it is probable that, in spite 

 of its indigenous appearance in America, it has become 

 naturalized after being accidentally transported thither. 



The genera of the vegetable kingdom, although 

 usually composed of several species, are often confined 

 to a single region. It follows, that the more species 

 included in a genus all belonging to the same quarter 

 of the globe, the more probable it is that one of the 

 species, apparently indigenous in another part of the 

 world, has been transported thither and has become 

 naturalized there, by escaping from cultivation. This 

 is especially the case with tropical genera, because they 

 are more often restricted either to the old or to the new 

 world 



