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 [pg 202] 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 rather the resultant of the conditions given in the chemo-physical predispositions of the organism itself, and in the architecture of its parts, plus the stimulus.

Directly associated with this property of irritability is another form of spontaneity and freedom in living beings—the power of adapting themselves to changed conditions of existence. Some do not show this at all, while others show it in an astonishing degree, [pg 203] helping themselves out by new contrivances, so to speak. Thus the organism may protect itself against temperature and other influences, against injury, making damages good again by self-repairing processes, “regenerating” lost organs, and sometimes even building up the whole organism anew from amputated parts. The mechanical interpretation must here proceed in the same way as in dealing with the question of stimuli, applying to the development of form the same explanations as are there employed. And just because this domain does not lend itself readily to mechanical explanation, we can understand that confidence in the sufficiency of this mode of interpretation grows rapidly with each fresh conquest, when this or that particular process is shown to be actually explicable on mechanical principles. Processes of development or morphogenesis—which are among the most intricate and difficult—are attacked in various ways. The processes of regeneration, for instance, are compared with the similar tendencies observed in crystals, which when they are injured have the capacity of restoring their normal form. This capacity therefore obtains in the realm of the inorganic as well as among organisms, and is referred to the tendency of all substances to maintain a definite state of equilibrium, conditioned by their form, and, if that is disturbed, to return to a similar or a new state of equilibrium. Or, the procedure may be to reduce the processes of a developmental or morphogenetic category to processes of stimulation in general, and then [pg 204] it is believed, or even demonstrated, that chemo-physical analogies or explanations can be found for them.

Thus, for instance, it is shown that the egg of the sea-urchin may be “stimulated” to development, not exclusively by the fertilising sperm, but even by a simple chemical agent, or that spermatozoids which are seeking the ovum to be fertilised may be attracted by malic acid. These are “reductions” of the higher phenomena of life to the terms of a lower and simpler process of “stimulus,” that is to say, to chemotropism in the second case and something analogous in the first. A further reduction would be to show that the movement of the spermatozoids towards the malic acid is not a “vitalistic” act, much less a psychically conditioned one, (that is, conditioned by “taste,” “sensation,” and the voluntary or instinctive impulse liberated thereby), but is a chemo-physical process, although perhaps an exceedingly complex one. It would be another “reduction” of this second kind, if, for instance, the well-known effect of light on plants, which makes them turn their leaves towards it (heliotropism), could be shown to be due to more rapid growth of the leaf on the shaded side, which would lift up the leaf and cause it to turn, or to an increase of turgescence on the shaded side, and if it could be shown that the increase in either case was a simple and obvious physical process, the necessary consequence of the decreased amount of light.

It is obvious, and it is also thoroughly justifiable, that all attempts along these lines of interpretation should [pg 205] be undertaken in the first place in connection with the simplest and lowest forms of life. It is in the investigation of the “Protists,” the study of the vital phenomena of the microscopically minute unicellular organisms, that attempts of this kind have been most frequently made. And they follow the course we have just indicated; the “apparently” vitalistic and psychical behaviour of unicellulars (impulse, will, spontaneous movement, selecting and experimenting) is interpreted in terms of reflex processes and the “irritability” of the cell, and these again are traced back, like all stimulus-processes, to the subtle mechanics of the atoms.