278 ON BOTANICAL GEOGRAPHY. 



Emily. Are there any natural limits which separate 

 different regions in the same continent ? 



Mrs. B. There are, but they are less defined than 

 those separated by seas ; so that there is a greater mix- 

 ture of plants in these regions. Their natural limits in 

 continents are, for instance, either extensive sandy des- 

 erts, such as those of Sahara, which separate northern Af- 

 rica from Senegal, or chains of high mountains, which 

 oppose an insuperable barrier to the conveyance of the 

 seed by natural means ; or, again, vast salt-plains, which 

 prevent the germination of seeds. 



Emily. But are there not a variety of means by which 

 plants may be conveyed from one region to another ? 



Mrs. B. No doubt ; and that accounts for plants ap- 

 pertaining to different regions often being found growing 

 in the same. Rivers, for instance, and high winds, con- 

 vey seed from one country to another ; birds of passage 

 transport the seed on which they feed ; animals carry 

 them in their wooly or hairy coats ; and, finally, man con- 

 reys seed wherever he goes : sometimes voluntarily, as 

 corn and potatoes, which he has disseminated all over the 

 known world ; at other times, unintentionally. And it is 

 owing to this casual transport, that the plants, and even 

 weeds, of most of our villages, have found their way to 

 America. 



Caroline. Like Robinson Crusoe, when by shaking the 

 dust out of a bag, he produced a crop of corn. 



Mrs. B. Very true ; but men have even gone further, 

 and conveyed seeds from one part of the world to anoth- 

 er, much against their intention or inclination ; such as 

 the seeds of the wild poppy and corn-flower, which can 

 never be completely separated from the grains of corn. 

 But, independently of these emigrations, it must be con- 

 fessed, that there is a small number of similar plants ex- 

 isting in different regions, without the possibility of ex- 

 plaining how they could have been conveyed from one 

 region to another. 



Caroline. This is quite a new idea to me : I always 

 thought that a great number of the same plants were to be 



1497. What is said of natural limits in the same country 1 ? 1498. 

 What instances are named 1 1499. By what different means are the 

 seeds of plants carried from country to countryl 1500. What seeds have 

 been carried by men against their intention* 1501. What is said, 

 independently of of these emigrations, of the existence in different coun- 

 tries, of similar seeds'? 



