36 DEVELOPMENT AND HEREDITY. 



is liable to accumulate so much that every now and 

 then the configuration of one of the groups breaks 

 up, and this, whether it is in a state of strain or 

 not. We may in this case suppose that in every 

 second a certain proportion of these groups break 

 up, and assume configurations corresponding to a 

 strain uniform in all directions. 



" If all the groups were of this kind, the medium 

 would be a viscous fluid. 



" But we may suppose that there are other groups, 

 the configuration of which is so stable that they will 

 not break up under the ordinary agitation of the 

 molecules, unless the average strain exceeds a cer- 

 tain limit, and this limit may be different for different 

 systems of these groups. 



" Now if such groups of greater stability are dis- 

 seminated through the substance in such abundance 

 as to build up a solid frame work, the substance will 

 be a solid, which will not be permanently deformed 

 except by a stress greater than a certain given 

 stress. 



" But if the solid also contains groups of smaller 

 stability and also groups of the first kind which 

 break up of themselves, then when a strain is ap- 

 plied the resistance to it will gradually diminish as 

 the groups of the first kind break up, and this will 

 go on till the stress is reduced to that due to the 



