This 4th of July, do we know what we’re celebrating? Is it unity or diversity? It had better be both. Historians will tell you what biologists can now confirm. Diversity is the surest defense against both internal threats — viruses, genetic deficiencies — and external threats — predators, climate change. Diversity in the moment is in fact what ensures unity (or stability) over extended periods of time. a couple centuries isn’t very long for a biologist, but already historians agree that the American experiment of a constitutional republic form of representational democracy owes its longevity to its suppleness. It can bend and not break, because the rigid lines of nationalism never took hold here. We are many people becoming one people. That’s something worth being proud of. Happy Independence Day.