Answer: The circle of life
In "There was a child went forth every day," Whitman describes the childhood of a kid. The poem is believed to be autobiographical.
The child is described as experiencing many events and discovering many new objects. He goes out and he encounters the animals, the flowers, the rivers. Whitman argues that everything that the child experiences becomes a part of him that he will carry through life. As time passes, all of these objects will remain with him. Moreover, the child will eventually become the objects that he perceived: the flowers, the grass, the land. This illustrates the circle of life. The idea is that we come from nature, and we will return to nature.