One of the main themes that we encounter in Whitman's poem is that of diversity and humanity. Whitman discusses how humans are joined together in a long line between the past, the present and the future. A long line of characters lived, loved and died before Whitman was born. Presumably, many others will come after. The ideas, knowledge, experiences and dreams of all these people will be condensed and passed down in time to the present. This is the theme that is explored in the lines 7-16.
Verse 52 is a fitting conclusion to the larger work as it once more discusses how Whitman is only created through a combination of everything that came before him, as well as everything that has ever been connected to him. He tells us that he dissolves, and he joins the landscape that he loves. He is part of everything, and if we were to look for him, we would find him everywhere and nowhere.