It didn't matter in the end how old they had been, or that they were girls, but only that we had loved them, and that they hadn't heard us calling, still do not hear us, up here in the tree house, with our thinning hair and soft bellies, calling them out of those rooms where they went to be alone for all time, alone in suicide, which is deeper than death, and where we will never find the pieces to put them back together.
This quote, the book's last, represents the boys' final elegy for the girls they loved. Despite a conscious attempt to reconstruct the events of their adolescence over the course of the novel, the boys realize that they are no closer to understanding the reasons for the girls' suicides than they were in the announcement of Mary's death in the book's first line. As the boys themselves grow older, their "thinning hair" and "soft bellies" signal the gradual approach of death. They must deal not simply with the lack of insight into the girls, but with the disintegration of what little knowledge they have. These decaying "pieces" are both abstract bits of knowledge and the disintegrating artifacts of the girls' lives that the boys have carefully collected and catalogued. Thus, the decay of the boys' memory is mirrored both in the decay of their environment and in the decay of their own bodies, just as the decay of the Lisbon household was mirrored both in the girls' bodies and in the disintegration of the Lisbon property.
Saddened by this intrusion of the physical world, the boys in this passage systematically reject the physical categories of age and gender, which have informed so much of the book, as ultimately inconsequential. Instead, the boys nurse their unanswered love, mourning the selfishness of the girls, who disappeared without ever hearing their call or deigning to reply. Sure of the saving power of love, the boys must believe that their cries were never heard. They can never admit the possibility that their cries were heard and rejected, or heard and ignored, and that perhaps suicide is not simply the girls' ignorance, but their deliberate reply.