Plant your seed in the morning and keep busy all afternoon, for you don’t know if profit will come from one activity or another—or maybe both.
Don't misunderstand, I am not a Christian but merely cross reference to any source of good information.
If it is mentioned in the book of wisdom, "The Bible", (I believe all religions are based on the foundation of wisdom) thousand of years ago, it is important for us in this modern day to practice these wisdom.
It's a bit harder to apply this principle to an individual since we all usually have just one career. However there are things we can do to diversify our overall sources of income.
You can concentrate on developing "multiple streams of income." If you want to make money outside your job, check out 11 Great Ways to Earn More Money, 20 Ways to Make More Money, and Eight Unusual Ways to Create Cash.
Of course, this principle certainly applies to your investments. You need to diversify, diversify, diversify because, as we all know, asset classes don't move up and down in lock-step. Some go up, and some go down. A year later, others go up and some others go down. No one knows which ones will perform in advance. So you invest in many asset classes -- with an asset allocation that makes sense for you.