The evolution of behavior.
Evolutionary theorists not only point to the survival value of the present structure and function of an organism, they try to reconstruct earlier stages which should also have had survival value. An example of current interest is the flight of birds. Feathers may have evolved first as thermal insulation, but what about wings? Were they adaptations of forelimbs which first helped animals run faster or that helped tree animals leap from branch to branch or from branch to ground? (Even when a feature first evolved because of consequences quite different from those which explain its current survival value, a plausible early history is still needed.) Among the features to be explained in this way is behavior. The current survival value of reflexes and the released patterns of behavior studied by ethologists may be clear, but can we construct plausible sequences through which they could have evolved, with survival value at every stage? The first behavior was presumably simple movement-like that of the amoeba reaching out into new territory and hence increasing its chances of finding materials necessary for its survival. A plausible second step was sensing, as the result of which movement could take the organism away from harmful stimuli and closer to useful materials. The assignment of different organs to sensing and moving should have led to the evolution of connecting structures, and eventually to tropisms and reflexes. The released behavior patterns studied by ethologists also presumably evolved through increasingly complex stages. It is unlikely that many current instances occurred first in their present state as variations which were then selected by survival. In my paper "The Shaping of Phylogenic Behavior" (Skinner, 1975),