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?” 
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. 
Lepidium 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? 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 chickweed; 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, xvii. 
2 Royal Institution Lecture, April 12, 1878. 
