What is the theme of the poem "Lucy Gray" by William Wordsworth?

The poem "Lucy Gray" tells the story of an innocent child named Lucy Gray who lives far from society in a cottage on the moors. One winter day, Lucy's father sends her off to town with a lantern to meet her mother. Unfortunately, Lucy gets lost in a snowstorm and never reaches town. The next day, Lucy's parents search for her throughout the wilderness and end up following her footprints in the snow. Lucy's parents track her footprints to a wooden bridge where her footprints suddenly disappear. Lucy's body is never recovered, but some people still maintain that they see her spirit as they walk through the forest.
Two of the main themes throughout Wordsworth's poem concern nature and the loss of a loved one. Lucy not only lives away from society on the moors, but she also travels through the wilderness. It is suggested that she enjoys nature because people claim to hear her playfully whistling on her journey to town. Lucy is depicted as an innocent child who enjoys the natural environment. Also, Wordsworth examines the loss of a loved one throughout the poem. Lucy's parents are concerned about their daughter's well-being when she does not arrive in town. The next day they search through the forest and are devastated to discover that Lucy's footprints end on the bridge. 

William Wordsworth's "Lucy Gray" tells the story of an innocent young girl who is lost in the wild and never found again. The suggestion is that she dies, although at the end Wordsworth suggests Lucy might still dwell in the wilderness somewhere far from civilization. It's unclear whether he's implying that Lucy is still alive, or if he's merely suggesting that her spirit lives on in the natural world. 
"Lucy Gray" is part of Wordsworth's "Lucy" poems, a group that includes the famous "She Dwelt Among the Untrodden Ways." All of these poems meditate upon a similar theme: the death of some unimportant and unknown person who lived in relative isolation in the midst of natural beauty. In general, these poems depict a young girl called Lucy who lives in close proximity to the wild and is gradually absorbed by the wild lands she inhabits, either through death or simply by disappearing. "Lucy Gray" follows this theme, as it shows the young, innocent Lucy disappearing into the vast world of nature.