54 INVASION 



and he pointed out that plants are able to pass barriers to a 

 greater or less degree by reason of natural or artificial 

 means of transport, such as water, winds, animals and man. 

 Link (J 820:263) observed the descent of alpine plants into 

 river valleys, pointing out the agency of mountain streams, 

 and indicating also the activity of ocean currents in bring- 

 ing about transportation, Schouw's silence in regard to mi- 

 gration would be inexplicable in view of the fact that his 

 book is largely a treatise on distribution, were it not for a 

 chance statement (J823:I82) in which he speaks of "Die An- 

 hanger der Migrationstheorie", and a note (184) in which he 

 says : ' 'Diese Theorie ist in der neuesten Zeit von Mannern 

 als DeCandolle, Link, und von Buch vertheidigt worden". 

 Meyen ( J836:39) considered briefly the currents of the air and 

 ocean as transportive agents, but added little of importance 

 to Link's account. DeCandolle (1832:595) considered the 

 natural dissemination of seeds under the following heads : 

 fruits psevdospermes (achenes, etc.) in which the fructification 

 plays a special part by the elongation of the peduncle, 

 the opening of the involucre or the swelling of the re- 

 ceptacle, or the fruit is itself pappose, barbed, etc.; 

 fruits charnus {fleshy fruits); fruits capsulaires, and des plantes 

 hypocarpoff^es. Henfrey (1852:50) has discussed the agents 

 of dissemination in an interesting manner, though he did 

 not contribute any new matter, with the exception of a short 

 treatment of barriers. A. DeCandolle (1855:613) considered 

 the causes of transport in a most minute though somewhat 

 unsystematic fashion. His book is a mine of details, 

 especially in regard to cultivated species. He was the first 

 to point out that, as a rule, barriers are obstacles to natura- 

 lization rather than to migration. Darwin ( 1859:624) brought 

 forward in a chapter on the means of dispersal a large 

 number of experiments and observations relating to the 

 transport of seeds by ocean currents, birds and other 

 animals. Kabsch (1865:543) did not attempt a consideration 

 of all transportive agents, but he gave particular attention 

 to the action of water currents and of glaciers. 



