GEOGRAPHICAL DISTRIBUTION OF ANIMALS 975 
151. Species debarred by inability to maintain their ground. 
—Hxamples of the second class are seen in animals which 
man has introduced from one country to another. The 
nightingale, the starling, and the skylark of Europe have 
been repeatedly set free in the United States. But none of 
these colonies has long endured, perhaps from lack of adap- 
tation to the climate, more likely from severity of competi- 
tion with other birds. In other cases the introduced species 
has been better fitted for the conditions of life than the 
native forms themselves, and so has gradually crowded out 
the latter. Both these cases are illustrated among the rats, 
The black rat, first introduced into America from Europe 
about 1544, helped crowd out the native rats, while the 
brown rat, brought in still later, about 1775, in turn practi- 
cally exterminated the black rat, its fitness for the condi- 
tions of life here being still greater than that of the other 
European species. 
Certain animals have followed man from land to land, 
having been introduced by him against his will and to the 
detriment of his domestic animals or crops. To many of 
these the term vermin has been applied. Among the ver, 
min or “animal weeds” are certain of the rodents (rats, 
mice, rabbits, etc.), the mongoose of India, the English 
sparrow, and many kinds of noxious insects. Of all the 
vermin of this class few have caused such widespread de- 
struction of property as the common European rabbit intro- 
duced into Australia. The annual loss through its presence 
is estimated at $3,500,000. 
It often happens that man himself so changes the en- 
vironment of a species that it can no longer maintain it- 
self. Checking the increase of a species, either by actually 
killing off.its members or by adverse change in its sur- 
roundings, is to begin the process of its destruction. Cir- 
cumstances become unfavorable to the growth or reproduc- 
tion of an animal. Its numbers are reduced, fewer are 
born each year, and fewer reach maturity, it grows rare, 
