Final answer:
The universal theme derived from Colonel Killigrew in 'Dr. Heidegger's Experiment' is that moderation is essential for a healthy life, as evidenced by Killigrew's decline due to his past excesses.
Step-by-step explanation:
From Colonel Killigrew's character in Nathaniel Hawthorne's 'Dr. Heidegger's Experiment', one can derive the universal theme that it is best to take all things in moderation. Colonel Killigrew's indulgent past, which involved numerous pleasures and excessive living, led to a current state of poor health and regret. This demonstrates the consequences of a life lacking moderation and supports the theme that moderation is key to a holistic and healthy life. The story implicitly cautions against the excesses that Colonel Killigrew once embraced and showcases the rejuvenation that he experiences, though temporarily, when given the chance to become young again—a chance that serves as a metaphor for the possibility of redemption and change.