26 GENERAL PHYSIOLOGY 



contributions to the physiology of the cell, and in our own time, 

 from this side especially, the physiology of reproduction, fertili- 

 sation, development, and heredity has been taken away from 

 physiology proper, and developed into a fruitful and independent 

 subject. 1 



The comparative method has not been employed in physiology 

 since Johannes Miiller's time, unless the few researches that have 

 been conducted upon other animals than the usual dogs, rabbits, 

 and frogs are to be considered as comparative. 



Plant physiology, however, has developed quite independently 

 into a nourishing science ; and the distinguished labours of 

 Hofmeister, Nageli, Sachs, Pfeffer, Strasburger, Berthold, and 

 others have made this in recent times the most complete branch 

 of physiology. This is due partly to the fact that all vital 

 relations are much simpler and more easily surveyed in plants 

 than in animals, and partly to the fact that plant physiology has 

 made use of certain acquisitions of science that have thus far 

 found little or no application to the physiology of animals. 



There are three of the greatest discoveries of this century, from 

 the further expansion of which physiology is justified in still 

 expecting great results. 



One of these is the law of the conservation of energy, which was 

 definitely expressed by Robert Mayer (1814-1878), and was estab- 

 lished most comprehensively by Helmholtz. Modern chemical 

 investigations had led to a recognition of the law of the conservation 

 of matter, by showing that the quantity of matter, of atoms, in 

 the universe is constant, and that the smallest atom cannot by 

 any agency be destroyed or recreated. The law of the conservation 

 of energy expresses the same fixedness for the sum of the energy 

 of the universe. Energy, like matter, can be neither destroyed 

 nor recreated ; when it seems to appear or disappear, it merely 

 passes from one form into another. Among the recognised 

 forms of energy two varieties are distinguished : energy of 

 motion, or kinetic energy, when power is in action, i.e. is pro- 

 ducing motion ; and energy of position, or potential energy, 

 when it is latent but under certain conditions can come into 

 action. Thus, e.g., the potential energy that was produced in the 

 Carboniferous age by transformation of the kinetic energy of the 

 sun's rays through the activity of plants and was stored up as 

 chemical affinity in vast strata of coal, passes over into heat 

 upon combustion of the coal. The heat is transformed by steam 

 engines which are heated by the coal, into the energy of 



1 Rdsumes of what has been accomplished in this field are given by the following 

 books: Die, Zelle und die Gewebe, by O. Hertwig (1892) [authorised English trans- 

 lation, The Cell: Outlines of General Anatomy and Physiology, 1895]; Gesammelte 

 Abhandlungen iiber EntwicUungsmechanik, by W-. Roux (1895); La structure du 

 protoplasma et les theories sur Vheredite, etc., by Yves Delage (1895) ; [and The 

 Cell in Development and Inheritance, by E. B. Wilson (1896)]. 



