I am prompted to write this now because this week I got a Darwin £2 coin in my shopping change.
The coin was struck by the British Mint as a limited edition in 2009. It depicts a profiled head of the great man face to face with a chimpanzee.
Although legal tender, the Darwin £2 coin is scarce in small change.
In fact, "Its rarity means that, in mint condition it is worth three or four times its face value. Even a used coin is worth double its tender value. The price acceleration means that it well worth seeking out as an investment."
Another Darwin coin was issued in 22 carat gold. This is not in circulation although it, too, is legal tender. Most are in the hands (or rather safes) of collectors.
As for the £10 note, Charles Darwin has been on it for very many years.