The Substance Which Is AUve in Plants 145 



quired by plants and animals in the course of their evolution 

 has its seat in a special chemical constituent of the protoplasm. 

 On this view, evolution, physiologically considered, depends upon 

 chemical experunentation, so to speak, in the protoplasm, and 

 follows step by step on the successful formation of new chemical 

 compounds. But let the reader beware of accepting this sug- 

 gestion as knowledge; it is merely a speculation, but one of those 

 which, in science, it is legitimate to throw out ahead as a tem- 

 porary guide to further investigation. 



In common with all other substances in Nature, protoplasm 

 thus possesses its physical and chemical properties. But in ad- 

 dition it possesses another set not found in other substance; 

 and thereupon depend its powers to do the remarkable things 

 that it does. These may be termed its physiological or vital 

 properties, which are as follows; — the property of metabolism, or 

 power of causing orderly chemical changes within itself, including 

 photosynthesis and respiration, and the other changes recorded 

 in our chapter devoted to that particular subject: the property 

 of conduction, or power to transport substances in definite paths 

 through itself, including absorption, transfer, and excretion: the 

 property of growth, or power to incorporate new material and to 

 increase in size at special places : the property of division, or power 

 to separate portions of its own substance, the basis of reproduc- 

 tion: the property of mobility, or power to cause definite move- 

 ments of its own substance, the basis of protoplasmic streaming 

 and locomotion: the property of irritability (sensitivity), or power 

 to respond advantageously to various stimuli. This enumeration 

 of the physiological properties of protoplasm reads like the table 

 of contents of a book on physiology, — and it ought to, because 

 physiology is nothing else than a study of the properties of proto- 

 plasm. And here is a point of importance. Just as the physical 

 properties of any substance are believed to reside in certain ulti- 

 mate structural units, which are the smallest portions into which 

 that substance can be divided and still retain those properties, 



