Table 1 Summary of ideas from the early key players that have shaped our current concept of the body plan

Aristotle

384–322 BC

Unity of plan

Describes the structural plan used to classify animals based on the ladder of nature

Buffon

1707–1788

Unity of the type

States that animal forms descended from an idealized archetype

Cuvier

1769–1832

Function determines form

Divides animals into groups based on the function of their anatomical traits

Geoffroy

1722–1844

Form determines function

Demonstrates the relationships among animals based on the similarities of form

von Baer

1792–1876

Embryological concept of body plan

Based on his four laws of development, von Baer showed that types reflect the structural organization of embryos rather than that of adults

Owen

1804–1892

Archetype

Returns to an animal classification system based on a divine, idealized form

Darwin

1809–1882

Descent from a common ancestor

Replaced the common archetype used to define the body plan concept with common ancestry

Haeckel

1834–1919

Biogenetic law

Concept that individual development repeats the evolutionary history of the adult morphology of its ancestors