METHODS FOR PROVING ORIGIN OF SPECIES. 9 



by consulting floras, works upon specks in genera], 

 or herbaria, we ought to be able to solve it easily in 

 each particular case. Unfortunately it is, on the contraiy, 

 a queistion which demands a special knowledge of botany, 

 especially of geographical botany, and an estimate of 

 botanists and of collectors, founded on a long experience. 

 Learned men, occupied with history or with the inter- 

 pretation of ancient authors, are liable to grave mistakes 

 when they content themselves with the first testimony 

 they may happen to light upon in a botanical work. 

 On the other hand, travellers who collect plants for a 

 herbarium are not always sufliciently observant of the 

 places and circumstances in which they find them. 

 They often neglect to note down what they have 

 remarked on the subject. We know, however, that a 

 plant may have sprung from others cultivated in the 

 neighbourhood ; that birds, winds, etc., may have borne 

 the seeds to great distances; that they are sometimes 

 brought in the ballast of vessels or mixed with their 

 cargoes. Such cases present themselves with respect 

 to common species, much more so with respect to culti- 

 vated plants which abound near human dwellings. A 

 collector or traveller had need be a keen observer to 

 judge if a plant has sprung from a wild stock belonging 

 to me flora of the country, or if it is of foreign origin. 

 When the plant is growing near dwellings, on walls, 

 among rubbish-heaps, by the wayside, etc, we should be 

 cautious in forming an opinion. 



It may also happen that a plant strays from cultivar 

 tion, even to a distance from suspicious localities, and 

 has nevertheless but a short duration, because it cannot 

 in the long run support the conditions of the climate or 

 the struggle with the indigenous species. This is what 

 is called in botany an adverUive species. It appears 

 and disappears, a proof that it is not a native of tiie 

 country. Every flora oflers numerous examples of this 

 kind. When these are more abundant than usual, the 

 public is struck by the circumstance. Thus, the trrops 

 hastily summoned from Algeria into France in 1870, 

 disseminated by fodder and otherwise a number of 



