Checkbox check all is not working correctly

I have a form field where I have two fields one containing countries and other containing memberships and both have check boxes. The problem is that with the code I am using to Select All the countries (check/uncheck all the check boxes for countries) also checks or unchecks the memberships field. I want to differentiate between the two so that I only check countries and not the check boxes in the membership field.

Here is the code I am using:

$("#checkCountries").click(function(){
  $('input:checkbox').not(this).prop('checked', this.checked);
});

Here, input:checkbox is making the code to work on all the checkboxes in the page. I think I need to change this to make it work specifically for that particular field (countries) only. Please help.