6^6 THE POPULAR SCIENCE MONTHLY, 



obscure carbon compound we call chloropbyl — 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 irritabil- 

 ity. Dutrochet follows the older botanists when heliotropism is 

 presented for his consideration, and attributes the whole phenord- 

 enon to the creative intelligence behind the organism. Now Hart- 

 mann, 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 Marti- 

 neau 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 for- 

 getting 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 pos- 

 sible. As Sachs defines it, " it is the mode of reaction to stimuli 

 which is peculiar to living organisms." It is what Herbert Spen- 

 cer 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 atten- 

 tion 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 ex- 

 hibit upon the slightest disarrangement of any of its components. 

 "While the manifestations of irritability are by no means condi- 

 tioned 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 origi- 

 nal impulse ; but this impulse itself is a characteristic of proto- 

 plasm. 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. 



