In living things new ones must be

I think I know where you're going with this!

"In living things, new ones must be born to replace the old ones that die."

This is a fundamental concept in biology, known as the "principle of replacement" or "principle of regeneration". It states that in order for a population of living organisms to maintain its size and stability, new individuals must be born or produced to replace those that die or are removed from the population.

This principle is essential for the survival and persistence of species, as it ensures that the population remains viable and can continue to evolve and adapt to its environment. Without this principle, populations would eventually decline and become extinct.

Is that what you were thinking?