Law of Genesis

Genesis 1:24 (King James Version)

And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so. 

God made man according to his ‘nature.’
God intended for man to reproduce more men with His spiritual image.

Originally, man was ‘of God’ and had God's spiritual image -- until sin broke that connection.

Mankind today is born with a sin image.  Sinning is part of our human nature.

God does a miracle in each person who chooses to accept Jesus as Lord and Savior.  The Bible says we become new creations.
God puts His spiritual image into us.  We call that receiving Jesus' Holy Spirit in our hearts and being born-again.

All things in life produce after their kind.   This includes our works (or fruit).
When we rely on the Holy Spirit, we produce good Kingdom fruit.
When we rely on our old sinful nature and our own efforts, we produce bad fruit.