obscure carbon compound we call chlorophyl—it also manifests what is named irritability. This irritability is a property of all living things, and is what distinguishes them from lifeless things. It is the one great difference between a monad and a crystal. Not only in the presence of light is irritability manifested by a living creature, but also when the influence of any other natural force is felt. It is, however, only with light that we have to do at present.
Perhaps, in the whole field of biological science there is not a more obscure subject than this very one of protoplasmic irritability. Dutrochet follows the older botanists when heliotropism is presented for his consideration, and attributes the whole phenomenon to the creative intelligence behind the organism. Now Hartmann, the great German pessimist, following in the footsteps of his master, Arthur Schopenhauer, attributes the twining of the wistaria and the bending away from light of the ivy-shoot to an unconscious will in nature; and teleologists like Paley or Martineau would make the whole field a basis for argument. To be compelled to call upon the first cause for what unquestionably lies within the domain of secondary causes is, of course, no less or more than a confession of ignorance, and one which the modern worker in science is always undesirous of making. Without forgetting that Newton showed himself both a great scientist and a great philosopher when he spoke of himself as but an explorer of the sea-shore while an ocean of undiscovered truth lay beyond, it seems certain that some scientific knowledge of irritability is possible. As Sachs defines it, "it is the mode of reaction to stimuli which is peculiar to living organisms." It is what Herbert Spencer had in mind when he defined life as a continual adjustment between internal and external relations; it is what Brooks has in mind when he calls life "education," and what Haeckel calls attention to when he describes life as "memory." Irritability is really, it would seem, little more than a tendency to abandon an unstable for a stable equilibrium, and may be compared to the tendency to fall which a complicated structure of blocks, for instance, will exhibit upon the slightest disarrangement of any of its components. While the manifestations of irritability are by no means conditioned upon protoplasm alone, they always have their origin in this compound. Mechanical structures of cell-wall and cell-contents act their part in modifying, transmitting, or translating the original impulse; but this impulse itself is a characteristic of protoplasm. Sachs compares the state of things in a plant-cell, before stimulus is applied, to the state of things in a locomotive upon the throttle-valve of which the engineer's hand is placed. A slight expenditure of force will set in motion a vast quantity of matter and may liberate a totally disproportionate amount of energy.
