DARWINISM AND EVOLUTION. 35 



Mobius proved that a pike requires three months to establish an 

 association of ideas. A glass partition in a tank separated some 

 minnows from a pike, which repeatedly dashed its nose against 

 the invisible wall, in fruitless efforts to catch the minnows. At 

 the end of three months the pike, having learned that its efforts 

 were of no use, ceased to continue them. The sheet of glass was 

 then removed ; but the now firmly established association of ideas 

 never became disestablished, for the pike never afterwards attacked 

 the minnows, though it fed voraciously on all other kinds of fish. 



But an ability to profit by experience rises in some animals to a 

 power of judgment. Dr. Rae, the naturalist, knew a dog in 

 Orkney which used to accompany his master to church on 

 alternate Sundays. To do so he had to swim a channel about a 

 mile wide ; and before taking to the water, he used to run about 

 a mile to the north when the tide was flowing, and a nearly equal 

 distance to the south when the tide was ebbing, almost invariably 

 calculating his distance so well that he landed at the nearest 

 point to the church. How the dog, says Dr. Rae, managed to 

 judge of the strength of the spring and neap tides at their various 

 rates of speed, and always to swim at the proper angle, is most 

 surprising. 



CEREBRATION. 



A s regards vertebrates, it is a remarkable fact that the cells of 

 -^--*- the brain and of the spinal cord are developed from the 

 same portion of the egg, the upper embryonic layer, as the cells 

 which form the skin and the organs of special sense. It is 

 remarkable because, between these structures, derived from similar 

 and contiguous elements, but becoming finally so different and sO. 

 separate, the most intimate relations continue through life. 



Every sensory organ is connected by means of nervous filaments 

 with a portion of the brain ; so that when a change has been 

 wrought by some external force on any sensitive surface, a 

 molecular wave is transmitted to appropriate cerebral centres. 



Such a wave of energy may be diffused through the brain and 

 forthwith expend itself in exciting muscular movements. Or it 

 may pass into a state of dynamic tension — the cerebral cells 

 playing the part of storage-cells — and the molecular or molar 

 motions to be ultimately performed may be indefinitely deferred. 



