In Sarah Orne Jewitt's tale, one of the most prominent conflicts is the tension between flesh and the spirit. She leaves it to the reader to decide which is the better path. This tension, as the critic you point to identifies, speaks to the increasing pull of money vs. preservation of the natural world and the old way of life.

Here is an excerpt regarding that conflict from the "Themes" page on Jewitt's novel at eNotes. You can learn more about this particular conflict, as well as the conflicts of coming of age (rites of passage), and many other literary elements by following the link below.

"When an appealing ornithologist comes to the Maine woods, young Sylvia must decide whether to please her new friend by showing him the nesting place of the heron he wishes to kill for his collection, or remain loyal to her animal companions. Although the nine-year-old girl would never consider her situation in these terms, the decision Sylvia must make is the choice between flesh and spirit—between earthly human pleasures and the natural world. The narrator states the conflict in a sigh directed at the reader: ‘‘Alas, if the great wave of human interest which flooded for the first time this dull little life should sweep away the satisfaction of an existence heart to heart with nature and the dumb life of the forest!’’

The hunter’s presence represents...fleshly desire.
(H)e offers Sylvia ten dollars if she will betray the heron. Although the sum seems to mean little to him, for Sylvia it is a great temptation: ‘‘He can make them rich with money; he has promised it, and they are poor now.’’ When he first offers the money, her head swims in confusion as she thinks of all she might buy. She is dazed and confused for the rest of the story, until the moment she decides not to tell the secret.