"Though developed for a particular adaptive function, an organism may eventually use a structure or attribute for a different, or exaptive, purpose. Penguins, for example, use wings to facilitate swimming, not flying."

http://www.galafilm.com/afterdarwin/english/glossary/exaptation.html

"The regular course of events seems to be, that a part which originally served for one purpose, becomes adapted by slow changes for widely different purposes."

Darwin, C. (1862)

Click: Stephen J. Gould and Elizabeth Vrba

Equifinality:

"a condition in which different initial conditions lead to similar effects."

http://pespmc1.vub.ac.be/ASC/EQUIFINALIT.html

Click: http://www.panarchy.org/vonbertalanffy/systems.1968.html 

"The first is the principle of equifinality. In any closed system, the final state is unequivocally determined by the initial conditions: e.g. the motion in a planetary system where the positions of the planets at a time t are unequivocally determined by their positions at a time t°.
This is not so in open systems. Here, the same final state may be reached from different initial conditions and in different ways. This is what is called equifinality."

Ludwig von Bertalanffy

http://www.panarchy.org/vonbertalanffy/systems.1968.html

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