The root of human virtue seldom bears
Like branches; and the Giver wills it so,
That men may know it is His gift, not theirs.
 
[Rade volte risurge per li rami
l’umana probitate; e questo vole
quei che la dà, perché da lui si chiami.]

Dante Alighieri (1265-1321) Italian poet

The Divine Comedy [Divina Commedia], Book 2 “Purgatorio,” Canto 7, l. 121ff (7.121-123) (1314) [tr. Sayers (1955)]
    (Source)

Dante noting that the sons of great kings rarely measure up to their fathers, a reminder from God that those who would be great must seek His blessing, not rely on their heritage.

(Source (Italian)). Alternate translations:

Rarely into the branches of the tree
Doth human worth mount up; and so ordains
He who bestows it, that as his free gift
It may be call’d.
[tr. Cary (1814)]

Rarely shoots merit up into the boughs,
Or human worth; and such the will of Him,
That from the Donor they should seem to come.
[tr. Bannerman (1850)]

Not oftentimes upriseth through the branches
The probity of man; and this He wills
Who gives it, so that we may ask of Him.
[tr. Longfellow (1867)]

Seldom rises human goodness through the branches; and this wills He who gives it in order that from Him it may be claimed.
[tr. Butler (1885)]

But rarely in the branch again is grown
Our human excellence, so willeth He
Who gives it, that the boon be called His own.
[tr. Minchin (1885)]

Rarely doth human goodness rise through the branches, and this He wills who gives it, in order that it may be asked from Him.
[tr. Norton (1892)]

Rarely doth human probity rise through the sons branches:
and this he wills who giveth it,
so that it may be prayed for from him.
[tr. Okey (1901)]

Rarely does human worth rise through the branches, and this He wills who gives it, that it may be sought from Him.
[tr. Sinclair (1939)]

Full seldom human virtue rises through
The branches; and the Giver wills it so,
That they to him for such a gift may sue.
[tr. Binyon (1943)]

Rare is the tree that lifts to every limb
the sap of merit -- He who gives, so wills
that men may learn to beg their best from Him.
[tr. Ciardi (1961)]

Rarely does human worth rise through the branches, and this He wills who gives it, in order that it may be asked from Him.
[tr. Singleton (1973)]

Not often does the sap of virtue rise
to all the branches. This is His own gift,
and we can only beg that He bestow it.
[tr. Musa (1981)]

Rarely does human worth rise through the branches;
That is the will of him whose gift it is,
So that it should be matter for petition.
[tr. Sisson (1981)]

How seldom human worth ascends from branch to branch,
and this is willed by Him who grants that gift,
that one may pray to Him for it!
[tr. Mandelbaum (1982)]

Seldom does human probity rise up through the branches, and this is willed by him who gives it, that it may be attributed to him.
[tr. Durling (2003)]

Human worth rarely increases through its branches: and this He wills who creates it, so that it may be asked for of him.
[tr. Kline (2002)]

It seldom happens that man’s probity
will rise through every branch. He wills it thus,
so, given from beyond, it’s known as His.
[tr. Kirkpatrick (2007)]

Rarely does human worth rise through the branches.
And this He wills who gives it,
so that it shall be sought from Him.
[tr. Hollander/Hollander (2007)]

Goodness rarely flows to the spreading branches
Of a family tree, for God who gives it decrees
That since the gift is His, humans must ask it.
[tr. Raffel (2010)]