The common law is said to grow case-by-case while preserving overall coherence. We test this claim with the citation network of all U.S. federal opinions published from 1790–2024 (760,000 cases, 8.3 million edges). An unsupervised community-detection algorithm, blind to courts or topics, recovers the judiciary’s familiar hierarchy and exposes cross-cutting thematic links. Four patterns emerge.

First, doctrinal experi-mentation is constant: hundreds of small citation clusters arise each decade, and over 90% disappear within twenty years.

Second, every major field—e.g. administrative law, civil procedure, intellectual property—traces a single citation lineage that endures for at least a century, with newer competing doctrines arising over the 1900s.

Third, continuity is uneven: nineteenth-century opinions on constitutional structure, federal-ism, and intellectual property remain deeply integrated with modern cases, whereas most other subjects form stable communities only after the 1960s–1970s.

Fourth, per-sistence depends on early doctrinal foundations: legal communities that gain an early foothold across many courts tend to endure, rather than being sustained by dominance by a few large courts or by the Supreme Court.

These results suggest that persis-tent doctrines anchor a self-organizing common-law network, allowing federal courts to absorb fresh opinions without losing intelligibility.