70 ILCS 1210/1
All offices and places of employment, other than those specifically exempted by the provisions of section 11 of this Act, in any park district (including under this designation any one or more towns having under Acts adopted by such town or towns, corporate authorities authorized by law to levy taxes for the maintenance of a public park or public parks) having 500,000 or more inhabitants residing within its territorial limits shall be classified and filled in the manner hereinafter provided for and not otherwise. Whenever any park district not now containing within its territorial limits 500,000 or more inhabitants shall have attained such number of inhabitants as shown by any census hereafter taken, thereupon this Act shall become applicable to such park district on the first day of July next succeeding the completion of the taking of such census.
70 ILCS 1210/1