What Darwin suggested is essentially phylogenetic bracketing: looking at close relatives and assuming that shared traits would also have been in the ancestor. Use of outgroups is a negative version of this: pick distant relative which should not share traits with members of the group so it can be used as a baseline of comparison.
Work Step by Step
Here you should review the definitions and use of outgroups and bracketing in the chapter. You will readily see that these are directly connected to Darwin’s suggestion, especially the bracketing technique which has an almost identical definition.