ii 14 Physical basis of mental evolution. 



-occurs with the flow of the blood when an artery is 

 tied or becomes obstructed. A plant when growing 

 in a dark recess, bends itself towards the light as if it 

 preferred light ; and its roots adapt themselves to the 

 iforms of existing obstacles. A decapitated frog jumps 

 .away from a source of irritation, as if he still possessed 

 ^sensation, volition, and choice. A man seeking his 

 way through a crowd avoids the course in which the 

 >the throng is densest. The human mind also, chooses 

 .as it were, the easiest way of solving a problem, and 

 usually adapts itself to altered circumstances. 



The principle of evolution also operates both in 

 iphysical and mental actions, and is a result of that 

 of causation. Complexity of structure and function 

 'is evolved out of simplicity of composition and 

 ^property by plurality of causes and conditions. For 

 instance, many complex forms of crystals of ice 

 .are produced from water. Calcspar crystallizes in 

 more than one hundred varieties of form, (all derived 

 from an obtuse rhombohedron) under the influence of 

 ,a number of slightly different conditions of temper- 

 .ature, impurities in the solution, &c. The most com- 

 plex bodies are evolved out of the simplest, the bodily 

 frame of man himself (and that of other animals) is 

 constructed of less than twenty of the elementary 

 substances. The same simple substances are capable 

 -of yielding very different and more complex bodies 

 under different conditions ; thousands of different 

 ^chemical compouuds are composed of hydrogen, 

 oxygen, and carbon only. In the development of 



