The problem fell back after all on the intermediate zoophytes which,
instead of separating the two kingdoms, resulted in the opposite, the
strengthening of the unifying ties. Trembley's hesitancy in the course of his
experiments on the hydra may be recalled in this connexion. If reproduction
by budding, green colour and absence of specific excretory organs spoke for
its being a vegetable, contractility and digestion spoke in favour of its being
an animal, and the hydra renamed polyp by Reaumur became the typical
example of the zoophyte. It is under those circumstances that one of the
most resounding discoveries of the eighteenth century, Haller's irritability,
appeared to offer the decisive device for which the naturalists were looking.
Haller did not himself work on the problem, but his contemporaries used his
discovery. The importance and extent of Haller's irritability is a neglected
subject's and its implications both physiological and metaphysical in inverte-
brates are totally ignored.
Giovanni Francesco Cigna (1734--1790), one of the founders of the
Academy of Sciences of Torino and the author of a dissertation De irri-
tabilitate (1758), thought that Haller's irritability would enable naturalists to
distinguish between the two kingdoms: "At last, irritability has provided us
with a new landmark for distinguishing vegetables from animals. The
former are deprived of all trace of irritability, excepting the sensitive plant