44 Causes and Course of Organic Evolution 



Such considerations as the above cause us now to close the 

 present chapter with a comparison of inorganic energies in 

 relation to the five great physiological activities that have 

 generally been looked on as typical of and peculiar to organ- 

 isms. These five are irritability, nutrition, respiration, growth, 

 and reproduction. If direct evolutionary continuity has existed 

 in world processes, from the simplest to the most complex 

 combinations of energy and ether, these terms must either 

 have an arbitrary and artificial limitation, or — if our knowledge 

 untrammeled by our hereditary prejudices enables us — they 

 should be extended to include a wider range of phenomena 

 than has hitherto been accepted. The latter seems to be the 

 more accurate attitude to assume, and we shall now try to 

 ascertain how far and how appropriately such terms can be 

 applied to phenomena of the inorganic world. 



Here the writer may be pardoned if he suggests that the 

 treatment of organic activity in many current physiological 

 text-books is by no means helpful to a consistent and consecu- 

 tive study of the subject. Only when organic bodies are viewed 

 from the five successively ascending phases above indicated 

 do w^e secure a concrete picture of physiological continuity. 



Irritability has usually been accepted as a term peculiar 

 to the biologist's province. Without attempting to give or to 

 discuss the many current definitions of it, that which seems 

 alone to cover all conditions in the organic realm would be 

 "molecular response to environal stimuli, that may be visible 

 or invisible to the naked eye." Now" we do not need to adduce 

 endless instances to show that all material bodies conform to 

 such a principle. We would rather say that if we attempt 

 to limit the term to living bodies, mth our increasing knowledge 

 of physical chemistry, we are constantly landed in unending 

 inconsistencies, as we hope to show in later chapters. We are 

 compelled to accept it that living and non-living bodies are 

 alike irritable — the chemist says that they act and react — and 

 that the range of it may be from the infinitesimally sluggish 

 response by a crystal of silica locked up for millions of years 

 in some igneous rock, up to the delicate and rapid response of 



