POLICE POWER 21 
been given, and the general principles under which 
it operates may cause expansion or contraction of 
its application upon the same general subject un- 
der different circumstances. To prevent harm 
being done by unqualified or unprincipled practi- 
tioners of medicine or surgery, under police power 
the doctor may be compelled to present evidence 
of his fitness for the trust, and in order to carry 
out this provision the state may prohibit any per- 
son from attempting to treat the sick or injured 
animals of any species until he shall first have 
obtained a license. Under police power quaran- 
tines are established, the production and sale of 
foods are regulated, industrial enterprises are 
supervised, the construction of buildings is con- 
trolled, the breeding of flies, mosquitoes, rats and 
other pests is restricted, and trade operations are 
watched to prevent fraud. Under this power stat- 
utes are enacted by the state, and ordinances by 
the city. Sometimes it is exerted by an executive 
officer unsupported by any previous legislation, 
and it is recognized by the courts. It is a tre- 
mendous power, which may easily be abused, but 
its existence and recognition is of the utmost im- 
portance for the general good. 
12. Police Power Resides in the States. The 
Ninth and Tenth Amendments to the Federal Con- 
stitution have generally been interpreted as grant- 
ing to the individual states the exclusive right to 
use the police power, and under state regulation 
this power has been exercised by the towns and 
cities within its border. Under that interpreta- 
tion the nation has no authority to exert this 
power within the different states. Practically the 
