The political evolution manifested by increase of mass is political
aggregation. One of the laws of evolution at large is that integration
results when like units are subject to the same force or the like
forces; and from the first stages of political integration to the last
this law is illustrated. Likeness in the units forming a social group
being one conditioned to their integration, a further condition is their
joint reaction against external action: co-operation in war is the chief
cause of social integration. The temporary unions of savages for offence
and defence show the initiatory steps. When many tribes unite against a
common enemy, long continuance of their combined action makes them
coherent under some common control. And so it is subsequently with still
larger aggregates.