I recommend taking a look at Filtrify, the Single Category Select is possibly close to what I imagine you need, but you can play with all the other demos as well, to see what might work best for you. The Single Category Select example basically allows you to start with a large category and limit it to smaller categories. There example uses music instead of countries, and does it with a single dropdown menu, which may be a nicer user interface. They even have an option that uses Isotope.
As I said before, this is really more about your data and thinking of the easiest way to present it to the user. If you absolutely need data in one field to be a subset of data based on a selection from another field, then start with the Query Field field_type. If you just have a long list of data in a database, excel, xml, etc., that you are trying to improve user interaction with, then take a look at all the options. The JSFiddle option will give you a starting point for multiple pulldown menus based on user selection, but something like the Single Category Select by Filtrify may present a far better option for the user.