跡 means 'trace'

All four essays involve digging into matters, examining physical evidence of the past (including ruins, footprints, or even handwriting), making discoveries, drawing conclusions about what happened, and then taking action. That could mean closing the case on a crime, declaring a missing person legally dead and doling out inheritance money, reconstructing a historic site, or pursuing someone who has disappeared. All four essays reinforce the idea that although a living creature can vanish or die ridiculously fast, we all leave marks on this world.