Plant Migration 309 



rigorous animals at any rate would move southwards to escape them. 

 It would be equally the case with plants if no insuperable obstacle 

 interposed. This implies a mobility in plants, notwithstanding what 

 we know of means of transport which is at first sight paradoxical. 

 Bentham has stated this in a striking way : " Fixed and immovable 

 as is the individual plant, there is no class in which the race is 

 endowed with greater facilities for the widest dispersion.... Plants cast 

 away their offspring in a dormant state, ready to be carried to any 

 distance by those external agencies which we may deem fortuitous, 

 but without which many a race might perish from the exhaustion of 

 the limited spot of soil in which it is rooted 1 ." 



I have quoted this passage from Bentham because it emphasises 

 a point which Darwin for his purpose did not find it necessary to 

 dwell upon, though he no doubt assumed it. Dispersal to a distance 

 is, so to speak, an accidental incident in the life of a species. 

 Lepidmm Draba, a native of South-eastern Europe, owes its pre- 

 valence in the Isle of Thanet to the disastrous Walcheren expedition ; 

 the straw-stuffing of the mattresses of the fever-stricken soldiers who 

 were landed there was used by a farmer for manure. Sir Joseph 

 Hooker 2 tells us that landing on Lord Auckland's Island, which was 

 uninhabited, "the first evidence I met with of its having been 

 previously visited by man was the English duckweed ; and this I 

 traced to a mound that marked the grave of a British sailor, and 

 that was covered with the plant, doubtless the offspring of seed that 

 had adhered to the spade or mattock with which the grave had 

 been dug." 



Some migration from the spot where the individuals of a species 

 have germinated is an essential provision against extinction. Their 

 descendants otherwise would be liable to suppression by more vigorous 

 competitors. But they would eventually be extinguished inevitably, 

 as pointed out by Bentham, by the exhaustion of at any rate some 

 one necessary constituent of the soil. Gilbert showed by actual 

 analysis that the production of a " fairy ring " is simply due to the 

 using up by the fungi of the available nitrogen in the enclosed area 

 which continually enlarges as they seek a fresh supply on the out- 

 side margin. Anyone who cultivates a garden can easily verify the 

 fact that every plant has some adaptation for varying degrees of seed- 

 dispersal. It cannot be doubted that slow but persistent terrestrial 

 migration has played an enormous part in bringing about existing 

 plant-distribution, or that climatic changes would intensify the effect 

 because they would force the abandonment of a former area and the 

 occupation of a new one. We are compelled to admit that as an 



1 Pres. Addr. (1869), Proc. Linn. Soc. 1868—69, pp. lxvi, lxvii. 



2 Royal Institution Lecture, April 12, 1878. 



