1） It is usually the case that
2） It is plain common sensethe more/lessthe 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 ofcan serve as a good illustration that