Individuals are born. They grow old. The phenomenon is independent of natural selection. The process seems to be dictated by biology not circumstance. There is plenty of data about to support the conclusion that every biological entity on earth is subject to the process.
Species emerge, they mature, they go into decline, they die.
I see advantages and disadvantages in offering an idea like this. Perhaps we could modify the exploration by invoking the metamorphosis analogy. I think that Darwin could have drawn on other possibilities as well. One possibility he might explore is the notion of threshold response. I've picked life cycle, metamorphosis, and threshold as naive analogy alternatives for a mind not bound to the certainty that variation is essentially unlimited.