VOL. 1, 5: The Economy of Nature. 75 
respects, organization, conditions of life, relation to each other, they 
differ so much that already in ancient time Aristotle arranged them 
in classes, and modern science talks in a rather unphilosophical way 
about higher and lower animals and plants. 
As a general rule the so-called higher forms of life exercise their 
influence by individuality, the lower ones chiefly by their numbers— 
the one endangers our interests by organs of destruction, the other 
by their unlimited powers of multiplying. 
Now as to what takes place when the original variety of organ- 
isms in an area is superseded by the exclusive cultivation of one or 
several organisms introduced by man: 
Each species has its antagonists, and these rise and multiply in 
proportion to the facilities offered to them. These facilities are of 
two kinds; first, an ample supply of food; second, protection against 
the species to which they serve as food themselves, and which we 
call, by a not quite exact expression, their enemies. 
As long as the ground is covered by a varied vegetation, inhab- 
ited by different kinds of animals, the enemies of the different spe- 
cies lose a great deal of their feeding time by searching and select- 
ing the peculiar plant or animal that serves them as food. As soon 
as a plant or animal occupies the ground to a more or less complete 
exclusion of other plants and animals, its enemies do not lose much 
time in the search for food, and will multiply at a more or less 
undesirable rate. The enemies of the species that formerly occu- 
pied the ground and are now excluded, will disappear at the same 
rate as the species disappear that formed their food when the orig- 
inal flora and fauna occupied the ground. 
In studying the multitude of phenomena developed during and 
after the change of a so-called wild state into the state of cultivation 
we find some the most striking developed in the insect world. 
At the same time it is indispensable to refer frequently to phe- 
nomena of a different order, because, with all her variety, nature is 
a unit, and it is only in text books that the citizens of the two king- 
doms may be studied in isolation. 
The first change produced by cultivation is an increase of the en- 
emies of the cultivated species. We have seen that nature opposes. 
any predominance that would end in monotony, so she has recourse 
to the same remedy used against the prevalence of the cultivated 
species and corrects her own remedy by raising an enemy to the 
enemy. ee 
