Any policeman or constable of any borough, township, or city, or any agent of any duly incorporated society or association for the prevention of cruelty to animals, is hereby authorized to destroy, or cause to be destroyed, any animal or animals in his charge, when, in the judgment of such policeman, constable, or agent, and by the written certificate of two reputable citizens called to view such animal or animals in his presence,--one of whom may be selected by the owner of the said animal or animals, if he shall so request,--such animal or animals appears to be injured, disabled, diseased past recovery, or unfit for any useful purpose.