What is the symbolism in Henry James's novel The Wings of the Dove?

As with all novels from James's so-called "major phase",
the symbolism of The Wings of the Dove is complex and
multi-layered. F. O. Mathiessen, one of the most important American critics of the first
half of the twentieth century, has written interesting pages on the novel's symbolism,
which I summarise here and that you can read in full following the second link
below.

The image of the dove evokes images of purity and
innocence. This is certainly one ways in which the narrator sets up an opposition
between Milly Theale and the more manipulative characters such as Kate, her aunt Maud
(who is compared to an eagle with "gilded claws") and her lover Merton. It is Merton who
first sees Milly as a "Christian maiden, in the arena, mildly, caressingly martyred" by
"domestic animals". Yet, the narrator immediately complicates the symbolism quoting
Milly's own self-description of having used "the wisdown of the
serpent" to find the doctor that can best address her illness. In a
later scene in Venice, the image of the dove itself is shown as not only denoting
innocence, but also ability to conceal feelings and the truth. Finally, the whiteness of
the dove is also linked to Milly's pearls thus symbolizing the power that her wealth
gives her. The wings are also used as a symbol of protection.