The paintings are in inverse order, chronologically and spiritually. Morris Grave, the painter, is an exemplar of a pilgrimage we might all hope to take from a reality that is symbolized to one that is actual.
The bird that sings in the moonlight is a beautiful expression both of itself but primarily as a symbol (or interlocking set of symbols) that with diligence and awareness can be read out of the picture - song, bird, moonlight.
The Evening Primrose is simply utterly itself: imaging reality in its own uniqueness - transcendence is immanent in the particularities of a flower seen its its 'suchness'.
Graves is quoted as saying that he had surrendered 'religion' as so much 'blah' - not because, I think, it is inauthentic but because it is inadequate: a set of signs pointing to the way that is not the moon. The primrose is simply a primrose at evening itself yet transparent to all things. Seeing a primrose as it is means seeing all as it is: that everything that lives is holy.
Graves was a serious student of both Japanese aesthetics and, more importantly, Vedanta, and in that latter study, rightly realized that 'maya' - the divine play that weaves the world in consciousness - is not (as it is often translated) 'illusory' but, if seen aright, illuminating. God dances into form and 'you' can dance into seeing/participating in its revealing beauty. It remains 'illusory' only to those (all of us most of the time) who sit on the stiff back chairs around the dance floor, worrying about our steps, our style, our partners (bound in that separating self-consciousness that can be heightened, rather than dissolved, by the 'blah' of religion).