72 PLANT MIGRATION 
seeds and at once flying off in a straight line at, say, 
50 miles per hour could not convey seeds more than 
150 miles. Secondly, many observations show that 
on migration birds generally travel with empty 
stomachs and clean plumage and feet. It is clear, 
therefore, that, as in the case of wind dispersal, we 
must look to exceptional circumstances, not normal 
conditions, to provide opportunities for long journeys 
on the part of seeds. But for the transfer of seeds 
from France to England, for instance, or from 
England to Ireland, it is clear that birds furnish a far 
more efficient medium than wind or water. In one 
important particular, dispersal by animals has a great 
advantage over dispersal by wind—that it is practi- 
cally independent of the weight of the seeds. Thus, 
the heaviest of British seeds, the acorn, is carried 
about by Rooks, just as the hazelnut is scattered by 
Squirrels, or a head of Burdock fruits by a passing 
sheep. 
Having thus arrived at some idea of the high 
efficiency for dispersal of many kinds of seeds, it is 
with some little surprise that we observe—as we may 
on any country walk—that the plants which arise 
from these are in general no more abundant or more 
widely distributed than others which possess seeds 
devoid of any apparent advantages in this respect— 
seeds which cannot fly nor float, nor cling to a passing 
creature, and which are not eaten to any extent by 
birds so far as observation goes. The truth is, we 
have to remember, as emphasized in a previous 
chapter, that the world is already densely populated 
by plants, all of which survive by reason of their being 
specially fitted for their several habitats. They have 
