262 THE FITNESS OF THE ENVIRONMENT 



Viscosity. 



Torsion. 



The Velocity of the Molecules of Gases. 



Hardness. 



Magnetism. 



The Velocity of Light. 



Optical Activity. 



Friction. 



The Velocity of Sound. 



The Wave Length of Light. 



The Length of the Path of a Gaseous Particle. 



To these may be added the phenomena of 

 radioactivity, etc. 



It is clear that in the present state of 

 knowledge the consideration of most of these 

 properties is uncalled for. However, it may 

 perhaps be noted in passing that the com- 

 pressibility of water is remarkably small, that 

 of protoplasm even less. 1 Hence even great 

 changes in pressure do not readily damage the 

 organism, and, indeed, a frog's muscle appears 

 to function normally after undergoing a 

 pressure of 500 atmospheres. 2 Further, it is 

 of decided consequence for many reasons 

 that the optical properties of water are such 



1 Henderson and Brink, American Journal of Physiology, 

 XXI, 248, 1908. 



2 Henderson, Leland, and Means, American Journal of 

 Physiology, XXII, 48, 1908. 



