Dick York's absence as Darrin in Going Ape (1969) is explained by having him out on a business trip. He is supposedly returning from his trip, but has to stay there longer. Dick York missed several of the series shows near the end of the 5th season.
When Endora refers to Harry being related to Darrin, she used "Darwin" as a reference to Darwin's evolution theory as the chimp turned into "Harry".
Darrin's absence in previous episodes is explained as a business trip to Boston. In this episode Samantha says Darrin is returning from Canada.