The caps in the picture show the linear history (literally) of the development of the humble field cap during Imperial times.....
Starting with the Prussian blue feldmütze at the bottom this type of cap was in use from Napoleonic times and was much favoured as a comfortable alternative to the heavy helmets and other headgear of the time. From 1860 this form remained virtually unchanged, apart from the addition of the national cockade in 1897, until the introduction of the field grey uniform in 1907.
The second cap demonstrates the new colour and the quality cloth it is made from indicates it is a pre ww1 manufacture. It has seen war service, with the 1915 masking band attached and was a veterans bring back, acquired during 1916 by a British soldier.
Here is one with out the masking band for comparison.
1917 saw the introduction of the universal field cap with a field grey piping and cap band. The poor quality of the wool/ mix cloth is evident as the blockade of Germany and lack of raw materials hit home.
Final cap is a late war field cap that has been re-issued to the Reichswehr and saw service in the 1920's. The use of Imperial Army uniform was continued until stocks ran out.
Here they are next to a bit of art....................."Grau Tag" by Georg Grosz