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***