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……