Fixed or variable proportional relationship between different parts in one or various systems
It is already an old idea to try to compare the modifications of the proportions of various parts of the same system through time or the proportions of a corresponding part of various systems at the same moment of their growth.
The 19th century French paleontologist Georges CUVIER used the concept as a "principle of correlation between parts": "Any organized being forms a whole, a unified and closed system, whose parts correspond all to each other and converge towards the same definitive action… None of these parts may change without changes in the others too, and as a result, each separately considered shows and gives all the others" (CUVIER, 1812, as quoted by Stephen J. GOULD – 1983)
This idea has been reworked and considerably extended, specially in relation to evolution by d'Arcy W. THOMPSON (1916 – re-ed.1952) and later on by J. HUXLEY, alone (1945) and in collaboration with E.C.R. REEVE (1972, and in HUXLEY, 1945, p.267)