80 ALFALFA FARMING IN AMERICA. 
that it will succeed on sandy soil which is too light to produce 
profitable crops of other forage plants. 
Yellow trefoil or Hop clover (Medicago lupulina L.) is an 
annual species and may be distinguished from aifaifa by its more 
spreading habit, its shorter and broader tipped leaves, by its yel- 
low flowers, and, finally, by the fact that the pods are not coiled, 
as with alfalfa, although coiled to make a single incomplete spiral. 
These pods also differ from those of alfalfa in being black when 
ripe. This species has some value in moist regions, but is far 
inferior to alfalfa. 
Bur clover (Medicago denticulata Willd.) and Spotted Medic 
(Medicago arabica All.), like yellow trefoil, are also annual 
plants and have yellow flowers. They differ, however, from all 
of the above-mentioned species in having burry pods. Although 
grown in some localities, they are of little agricultural conse- 
quence. 
Bur clover inoculates land for alfalfa growing 
or vice versa. They carry the same bacteria on their 
roots. Mellilotus, or sweet clover, also uses the same 
bacteria. This fact is useful since it often enables 
us to get hold of inoculated soil, or to sow one of 
the inferior clovers as a forerunner of alfalfa for 
the purpose of inoculating the soil or of enriching it 
and storing it with humus. 
Types and Varieties Alfalfa is remarkably vari- 
able. One can go into a field sown all of one sort of 
seed and select in it a hundred plants, no two having 
very close likeness. Much can be done and will be 
done to select varieties having desirable character- 
istics. Already the Colorado and Kansas experi- 
ment stations are doing considerable in this line, 
while other stations not so well located are also at 
work, notably Ohio, Minnesota and North Dakota, 
and the Department of Agriculture at Washington. 
Natural selection,-or the law of the survival of the 
fittest, has done much to create types. For example, 
