Nature.

 
WHEN God made the world He gave to His creatures their nature. Thus the humble daisy still bears its blossoms of white stars, and the buttercup its yellow shining flowers. We do not ever find a buttercup flower growing out of a daisy root, or a daisy blossom upon a buttercup stem, and the Lord said, “Men do not gather grapes of thorns, or figs of thistles.” No cultivation can change nature; no effort can alter an Ethiopian’s skin, or a leopard’s spots!
Hence says the word, Ye must be born again. Ye must have a new life. The old is fallen, it cannot bloom in glory; it cannot enter the kingdom of God.