Forced movements and compensatory orientation

Loeb (1918) set out three groups of forced movements: circus movements to right or left, forward movement, and backward movement. He noted that the last is not always possible, and he left rolling around the long axis aside to simplify the discussion.⁵ His main claim concerned light. He held that its primary effect is a change in the tension of muscles. On this reading, heliotropic reactions that older observers took for voluntary acts are in reality forced movements.⁷ The argument treats behaviour as the mechanical result of unequal tension, not as choice.

Washburn (1908) summarised the older work on equilibrium. In vertebrates the static function was long known to reside in the ear, especially the semicircular canals. Results were not uniform. Sewall found that cutting the canals in the shark left balance intact, though operations on the vestibule and ampulle disturbed movement.⁴ Washburn judged that errors in method probably influenced the negative reports. Loeb cited Maxwell's rotation experiment on Phrynosoma. With the eyes closed, compensatory head motions appeared once the angular velocity passed a threshold.⁶

Jennings (1906) applied the question to Amoeba. He argued that the surface tension theory fails for those Amoebae that roll with all currents forward, since such rolling cannot be produced in a fluid drop by local changes in surface tension.¹ Rhumbler, by contrast, kept the theory for forms with backward currents, which meant different causes in different Amoebae. Jennings also found that Amoeba has no permanent axes, since front and back keep exchanging places.² He concluded that any orientation seen is a result of the direction of movement, not its cause.³ This was his objection to a strict tropism theory.

Later authors met the same phenomena from other directions. Lillie (1923) reported that rats subjected to prolonged rotation produced offspring with defects in equilibrium and a tendency to circus movements, which he cited as results of analogous kind to the antibody work of Guyer.⁸ Child (1924) took up the rolling Amoeba in terms of physiological gradients. He reported Hyman's finding that each pseudopod carries, for the time being, the graded differences of a simple physiological axis.⁹ Child cited Loeb's book among his references.