Every experiment must have two kinds of variables. It must have dependent variables, which are the variables that are being impacted. It must have independent variables, which are the variables that are affecting the dependent variables.
In this test, the dependent variables were variables having to do with development. Dependent variables were things like
economic specialization, social complexity, political organization, and material products... (p. 65 in the paperback edition of the book)
The dependent variables were variables of geography. These were things like the size of the islands, how isolated they were from other islands, and what resources they had.
Diamond uses these islands because only geographical variables change. The race and culture of the people on the islands is held constant while the islands' geography varies. This allows him to see the effect of geography on the dependent variables mentioned above.