Rhumbler, too, succeeded in explaining by “developmental mechanics” some of the apparently extremely subtle processes at the beginning of embryonic development (the invagination of the blastula to form the gastrula); by imitating the sphere of cells which compose the blastula with elastic steel bands he deduced the invagination mechanically from the model.(63) Here, too, must be mentioned Verworn’s attempts to explain “the movements of the living substance.”(64) “Kinesis,” the power to move, has since the time of Aristotle been regarded as one of the peculiar characteristics of life. From the gliding “amœboid” movements of the moneron, with its mysterious power of shifting its position, spreading itself out, and spinning out long threads (“pseudopodia”), up to the contractility of the muscle-fibre, the same riddle reappears in many different forms. Verworn attacks it at the lowest level, and attempts to solve it by reference to the surface tension to which all fluid bodies are subject, and to the partial relaxation of this, which forces the mass to give off radiating processes or “pseudopodia.” The mechanical causes of the suspension of the surface tension are inquired into, and striking examples of pseudopod-like rays are found in the inorganic world, for instance, in a drop of oil. Thus a starting-point is discovered for mechanical interpretations at a higher level.(65) Irritability. 3. A property which seems to be quite peculiar to living matter is irritability, or the power of responding to “stimuli,” that is to say, of reacting to some influence from without in such a manner that _the reaction_ is not the mere equivalent of the action, but that the stimulus is to the organism as a contingent cause or impulse setting up a new process or a new series of processes, which seem as though they occurred spontaneously and freely. Thus the sensitive plant _Mimosa pudica_ droops its feathery leaves when touched. Here, too, must be classed also all the innumerable phenomena of Heliotropism, Geotropism, Rheotropism, Chemotropism, and other tropisms, in which the sun, or the earth, or currents, or chemical stimuli so affect a form of life—plant, alga, or spore—that it disposes its own movements or the arrangements of its parts accordingly, turning towards, or away from, or in an oblique direction to the source of stimulus, or otherwise behaving in some definite manner which could not have been deduced or predicted from the direct effects of the stimulating factors. The upholders of the mechanical theory have attempted to conquer this vast and mysterious domain of facts by seeking to do away with the appearance of spontaneity and freedom, by demonstrating in suitable cases that these phenomena of spontaneity and the like would be impossible were it not that the potential energies previously stored up within the organism are liberated by the stimulus. Thus the effect caused is not equivalent to the stimulus alone, but is