The first is that, though the love of Nature was always less in him than his love of human nature, yet for the first half of his work it was so interwoven with his human poetry that Nature suggested to him humanity and humanity Nature.

And these two, as subjects for thought and feeling, were each uplifted and impassioned, illustrated and developed, by this intercommunion.

That was a true and high position.

Humanity was first, Nature second in Browning's poetry, but both were linked together in a noble marriage; and at that time he wrote his best poetry.
The second thing this chronological treatment of his Nature-poetry shows, is that his interest in human nature pushed out his love of Nature, gradually at first, but afterwards more swiftly, till Nature became almost non-existent in his poetry.

With that his work sank down into intellectual or ethical exercises, in which poetry decayed.
It shows, thirdly, how the love of Nature, returning, but returning with diminished power, entered again into his love of human nature, and renewed the passion of his poetry, its singing, and its health.