Fern functions as a transitional character. Her intital act of saving Wilbur is selfless but she is unable to maintain her selflessness. As she grows up her human interests overtake her childhood interests and she is forced to make a choice. So far as the novel is concerned, Fern foreshadows Charlotte but she is not Charlotte. Her willingness to be selfless for Wilber has its limits, its far to human limits to be sure.
Templeton also undergoes a transition as he saves Charlotte’s egg sac; although not entirely selfless, Templeton acts as selflessly as he possibly can act in this case. Engineering the transfer of Charlotte’s egg sac back to the barn is, for Templeton, an act of complete selfless behavior. There is nothing in it for him.