Gap between unlike cells
In total, twenty nine transitions suffice to simulate a binary counter, but there is still no source for the carries which operate it. Isolated bistable cells form its body; packed at their densest, quiescent cells must still separate them. Four generations are required to pass on a carry; therefore one glider every fourth cell is the closest feasible packing for gliders.
Thirty five transitions were not used in defining the basic counter, leaving them free to form an operating environment. Isolated cells and quiescent regions having been preempted by the counter, the remaining transitions necessarily describe the evolution of clusters.
Harold V. McIntosh