THE SEED 23 
the four agencies named, animals and wind are the most effec- 
tive, and the greater number of adaptations observed will be 
found to have reference to these. 
20. Involuntary dispersal.— The lower animals may be 
voluntary agents in a way, though not designedly so, as when 
Fie. 37.—Good quality of clo- Fic. 38.—Inferior quality of 
ver seed. clover seed mixed with ‘‘ screen- 
ings.” 
a squirrel buries nuts for his own use and then forgets the lo- 
cation of his hoard and leaves them to germinate; or when 
a jaybird flies off with a pecan in his bill, intending to crack 
and eat it, but accidentally lets 
it fall where it will sprout and 
take root. Both man and the 
lower animals are not only in- 
voluntary, but often unwilling 
agents of dispersal. Some of the 
most troublesome weeds of civili- 
zation have been unwittingly dis- 
tributed by man as he journeyed 
from place to place, carrying, 
along with the seed for planting 
his crops, the various weed seeds, 
or “screenings,”’ as these mixtures 
are called by dealers, with which 
they have been adulterated either through carelessness and 
ignorance, or from unavoidable causes. The neglected 
animals, also, that are allowed by short-sighted farmers to 
wander about with their hair full of cockleburs and other 
Fic. 39.— Dodder on red clover; 
showing how the seeds get mixed. 
