METHODS FOR PROVING ORIGIN OF SPECIES. 9 



by consulting floras, works upon species in general, ] 

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

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

 a question which demands a special knowledge of botany, 

 especially of geographical botany, and an estimate of 

 botanists and of collectors, founded on a long experiences^ 

 Learned men, occupied with history or with the inter- ^ 

 pretation of ancient authors, are liable to grave mistakes 

 when they content themselves with the lirst testimony : 

 they may happen to light upon in a botanical work. | 

 On the other hand, travellers who collect plants for a i 

 herbarium are not always sufiiciently observant of the ; 

 places and circumstances in which they find them, i 

 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 i 

 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 i 

 collector or traveller had need be a keen observer to ] 

 judge if a plant has sprung from a wild stock belonging ' 

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

 When the plant is growing near dwellings, on walls, i 

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

 cautious in forming an opinion. I 



It may also happen that a plant strays from cultiva- ; 

 tion, even to a distance from suspicious localities, and i 

 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 adventive species. It appears i 

 and disappears, a proof that it is not a native of the ! 

 country. Every flora offers numerous examples of this ! 

 kind. When these are more abundant than usual, the 

 public is struck by the circumstance. Thus, the troops 

 hastily summoned from Algeria into France in 1870, i 

 disseminated by fodder and otherwise a number of I 



