LitCharts assigns a color and icon to each theme in Messenger, which you can use to track the themes throughout the work.
Selfishness vs. the Collective Good
Youth, Memory, and the Future
Humans and Nature
Identity and Difference
Summary
Analysis
Just after nightfall, Matty realizes that Frolic is afraid of the dark. He picks Frolic up to comfort him and chooses a sheltered spot to spend the night, near the clearing where Matty first healed the frog. By the fourth night, all of Matty's food is gone. Matty isn't concerned; he knows how to find food in Forest. The journey is longer than normal, however, since Matty has to backtrack and go off course in order to post the messages of Village's closure. He finds what he thinks is evidence of the woman's group traveling through and wonders if she's gone back yet.
Note that Matty still believes that at least when it comes to him, Forest is a benevolent and guiding force. This suggests that Matty still doesn't understand that Forest is a reflection of what's going on in Village and will soon begin to reflect the fear and suspicion that's gripping his home. At this point, Matty believes his world is insular and confined to humans, rather than capable of influencing things on a grander scale.
Active
Themes
Matty begins to understand what Leader meant when he said that Forest was thickening. While Forest has always been familiar and welcoming to Matty, it now seems hostile. It's harder to catch fish, and some of the berries have black spots. Matty also notices poison ivy and that the trees seem to be creating more of a canopy. While this protects Matty from the rain, it makes Forest much darker. Forest also smells like decay. Matty starts to have nightmares and his throat and head begin to ache. To distract himself, he thinks of what he was like as a little boy. He was a fierce child and Kira's good friend.
The smell of decay in particular mirrors Ramon's illness and suggests that just like Ramon, Forest is dangerously ill. It's also contagious, given that Matty suffers just like Forest and Ramon do as he continues on his journey. Matty's choice to think of his friend Kira during this time reminds the reader that Matty still draws on his friendships and a sense of community to get him through difficult times.