I don't know if I totally agree with that. It really depends upon the individual unit. Some came from a parent active regiment but others were formed from all sorts of different sources. I would also caution that the numbers align with each other. Sometimes they do -- most times they do not. Things get pretty complicated when you overlay the political boundaries and the fact that regiments actually moved. Sometimes it is just so complex I cannot figure it all out. For instance, this morning I was looking at LIR 87. I do not understand what relationship existed between this Hessian regiment and Mecklenburg. The parent line is far too simplistic and it got worse as the war progressed. I think in the handbook of Imperial Germany there is a good example of JR 31. I think there is a reason why most sourcebooks disregard reserve and Landwehr formations -- it is just too hard and no one has figured it out yet. Great subject though! You also get into confusions about where the guard uniforms came from in units like RJR 202. Sometimes it's simple but sometimes...
I don't know if Joe ever got an answer to this question, but just in case not:
LIR 87 was mobilized with 4 battalions. The regimental staff and the I., II., and III. Btle. were raised in Mainz, and the IV. Btl. in Worms. There's your Hesse connection. The regimental staff was dissolved on 7 February 1915.
II/LIR 87 and III/LIR 87 with the attached IV/LIR 76, a Mecklenburg battalion from Rostock, were assigned to 6.Armee in December 1914. This only lasted a few months, and in March 1915, II/LIR 87 became III/RIR 17 (RIR 17 was originally a two-battalion regiment from the Rheinprovinz) and III/LIR 87 became III/RIR 30. IV/LIR 76 became III/RIR 69 in summer 1915. RIRs 17, 30 and 69 were all from the Rhineland, so it is likely that by this point, the original LIR 87 and LIR 76 battalions had lost most of their Hessians and Mecklenburgers and the replacements were from the more populous Rhineland.
The other two LIR 87 battalions, I/LIR 87 and IV/LIR 87, with the attached V/LIR 76 from Wismar in Mecklenburg-Schwerin, remained with Generalgouvernement Belgien and became Regiment Moß in early 1915. This was redesignated as the "new" LIR 87 in May 1915. I.Btl. remained the same, V/LIR 76 became the II.Btl., and IV/LIR 87 became the III.Btl. So "new" LIR 87 had two Hessian battalions and one Mecklenburg battalion.
Also, regarding the 2xx-series RIRs, RIRs 201, 202, 203 and 204 were all raised from Prussian Guard regiments.
As Glenn stated, RIR 202 was formed from the Ersatz battalions of GGR 1, GGR 2, and GGR 4
RIR 201 was formed from the Ersatz battalions of 2.GRzF, 4.GRzF, and the GFüsR
RIR 203 was formed from the Ersatz battalions of 5.GRzF, GGR 5, and GGR 3
RIR 204 was formed from the Ersatz battalions of 1.GRzF, 3.GRzF, and the LehrIR
I don't know if this makes things more or less confusing.