74 The Economy of Nature. [ZOE 
explained, but the fact is they are much more frequently heard on 
dark nights. The songs are always heard early in the mating sea- 
son, generally in March, and the most reasonable explanation, it 
seems to me, is that the birds are unusually excited at this time, 
and, awakening in the night, involuntarily break forth into song. 
That it is involuntary and unpremeditated, all who have heard this 
song will certainly agree. 
THE ECONOMY OF NATURE AS EXEMPLIFIED BY 
VEGETABLE AND ANIMAL PARASITES. II. 
BY Es Tis BRMR, 
The concatenation of cause and effect which produces the harmony 
in nature cannot be interrupted with impunity by interferences of 
violent character. The violent destruction of one evil almost always 
opens the door to a series of new evils, because the immense net- 
work of cause and effect called Nature being once disturbed it is im- 
possible to know at what point the given impetus will stop. 
It cannot be helped that our own interests interfere with the har- 
mony in nature. Neither cultivation of the ground nor raising ot 
stock is possible without disturbance of the original forms of or- 
ganic life, because the very object of all agricultural pursuits is to 
promote the prevalence of certain animals and plants useful to man 
to the more or less complete exclusion of the organisms previously 
occupying the same ground. 
We will first notice the immediate consequences of the interfer- 
ence in favor of the more desirable species at the expense of the 
original fauna and flora. 
Every species, animal and plant, is decimated or Jimited in its 
multiplication by other species, or else the variety of organic life 
now inhabiting and decorating the surface of this earth would have 
disappeared a long time ago, and would have made room to mon- 
otony. Each climatic girdle would be covered by a vegetation 
closely resembling cultivated grounds, each inhabited by one herb- 
ivorous animal, serving as food to one carnivorous animal. There 
is no place on this earth where such state of things actually has 
taken place, the agencies just mentioned preventing. 
Nature favors variety and opposes equality, the only things in 
which all organisms are equal, being birth and death. In all other 
