240 INITIATIVE IN EVOLUTION 



Facilitation. 



But of all these important reactions in nervous tissues none 

 bears so closely on the problem of the formation of reflex-arcs as 

 that of Facilitation. This is equivalent to the Law of Neural Habit 

 of the physiological psychologist, and is bound up with the highly 

 important Law of Forward Direction, which Professor Starling 

 says might as well be spoken of as the Irreciprocal conduction of 

 nerve-arcs. The Law of Forward Direction of sensori-motor arcs 

 is too well known to need here any description. But when this law 

 is taken into account the phenomenon of Facilitation is seen to 

 throw a strong light upon the earliest and rudimentary formation 

 of specialized nerve-fibres, reflex-arcs and Final Common Paths 

 leading to the effector glands or muscles. Facilitation is described 

 shortly by Professor Starling as follows. If the passage of a nervous 

 impulse across a synapse or series of synapses in the central nervous 

 system be too often repeated, fatigue is produced, and there is an 

 increase of the block at each synapse. If, however the stimulus 

 be not excessive and the impulse not too frequently evoked, the 

 effect of a passage of an impulse once is to diminish the resistance, 

 so that a second application of the stimulus provokes the reaction 

 more easily, and he adds that the result of summation of stimuli 

 is in fact in the direction of removal of block. When an impulse 

 has passed once through a certain set of neurones to the exclusion 

 of others it will tend, other things being equal, to take the same 

 course on a future occasion, and each time it traverses this path 

 the resistance in the path will be smaller. Education then is the 

 laying down of nerve-channels in the central nervous system, while 

 still plastic, by this process of Facilitation along fit paths, combined 

 with inhibition (by pain) in the other unfit paths. He makes the 

 important statement that Facilitation is of great interest in con- 

 nection with the development of " long paths " in the central 

 nervous system and, more especially with the acquirement of new 

 reactions by the higher animals. (Italics not in the original). 



Raw Materials of the Central Nervous System. 



The raw materials of higher central nervous systems are 

 furnished even in lowly Vertebrates by the neurones and their 

 processes, and the pathways into the grey matter by the " canalizing 

 force of habit " in the receptors and afferent fibres. Facilitation, 

 discovered in higher Vertebrates, such as dogs and cats, throws 

 backwards a light on the earliest struggles towards success and 

 integration among phyla, sub-phyla and smaller groups, and here 

 again the well-known may lead to the less-known. We may then 



