1） It is usually the case that ……

　　2） It is plain common sense——the more/less……the more/less……

　　3） The serious reality had taught us a lesson： not being environment  friendly will be avenged mercilessly.

　　4） As a proverb says/as is known to all/as a popular saying goes，……

　　5） I can think of no better illustration than an English poem/adage which goes like this：……

　　6） The old story of……can serve as a good illustration that……