198 BRITISH PLANTS 
6. Distribution of Seeds by Man.—Man, intentionally or 
otherwise, is not the least important of seed-distributors. 
Wherever his merchandize goes, the seeds of his native 
flora accompany it. For this reason a rich alien flora 
is generally developed in waste places about docks. 
Most of these strangers soon die out, but here and there 
some flourish and establish a permanent foothold in the 
country—e.g., the thistle in Australia. 
_ Of all the agents of seed-dispersal, the most efficient, 
excluding man, are the birds and the wind. If the seeds 
are heavy, the wind carries or drives them a very short 
distance ; explosive fruits seldom eject the seed beyond a 
few feet ; creeping fruits cannot travel far; animals do 
not generally wander far from their homes, physical 
barriers, natural enemies, and family ties seriously limiting 
their freedom. But the paths of the air are free, and 
oppose no obstacles to flight. Birds travel through the 
air much faster and much farther than animals traverse 
the land. So efficient is this method of dispersal, 
especially for large and heavy seeds, that plants whose 
seeds are so dispersed can afford to expend a large part 
of their substance in making large fleshy envelopes for 
a small number of seeds, instead of using up all the food 
in the production of as many seeds as possible. Migratory 
birds travel every year enormous distances, but as the 
migrations are north and south, the birds pass through 
latitudes which differ so widely in climate and seasons 
that the seeds they may bear with them are seldom 
capable of establishing a successful footing where they 
fall. Small light seeds, however, especially if equipped 
with floats, are carried a long way by the wind. These 
also have the advantage of being produced in great 
numbers. This accounts for the fact that plants possess- 
ing small wind-borne seeds, as a rule, travel faster along 
the roads of conquest than those which depend for their 
dispersion on birds. Moreover, the wind follows the 
lines of latitude more closely than birds, with the result 
that the dispersal of wind-borne seeds is effected through 
more uniform climates than seeds carried by birds in 
their migrations. 
