A philosophical position
Successful biological research in this century has had three doctrinal bases: mechanism (as opposed to vitalism), natural selection (trial and error, as opposed to rational plan), and historicity. This last is the recognition of the role of historical contingency in determining properties of the Earth's biota. The formation of the Earth was a unique event, and unique events have been altering evolutionary processes ever since, always keeping futures unpredictable from presents. As S. J. Gould ( 1989, p. 48) forcefully expressed it, if we could rewind the tape of evolutionary history to the remote past and play it again, it would turn out entirely different. The term evolution in its original sense of an unfolding or development, analogous to the development of an individual animal, is misleading ( Salthe 1989).
Mechanism implies that only physico-chemical processes are at work in an organism. Every vital function is performed by material machinery that can in principle be understood from a physical and chemical examination. The opposed doctrine of vitalism maintains that the observable machinery has but limited autonomy and is controlled by a purposive entity peculiar to living organisms. The history of much of biology in the nineteenth and early twentieth centuries can be viewed as a gradual retreat of vitalism and (final?) triumph of mechanism. Authors of biology textbooks that I read in the 1940s often presented vitalism as an idea with some adherents, but implied that mechanism was nearly universal among practicing professionals.
Vitalism was notably persistent in experimental embryology. Driesch ( 1929) and other influential researchers thought that the kind of goaldirected control shown by development could not arise from the observable structures themselves. The controlling principle must be found in vital forces ( Driesch's entelechies) found only in living organisms. Even welltrained scientists in Driesch's time had but limited experience with negative feedback control loops and no way of imagining today's elaborate