THE BUILDING OF REFLEX ARCS 241 



frame a legitimate hypothesis, or at least an ideal construction 

 of trials and errors and success, if those of lower levels were ever to 

 be introduced to the career of progress and achievement. But to 

 make good this claim it is necessary that it be based on the important 

 doctrine taught by Hughlings Jackson of the three (or more) levels 

 of sensori-motor arcs — those of the spinal or lowest, of the sensory 

 or intermediate, and those of the third or highest level, in which 

 the association-areas of the Primate brain are at once the means 

 and the title to his primacy, or headship of the sentient world. 

 The light of this doctrine guides the mind backwards to the frog- 

 stage of animal evolution with its highly organized congenital 

 system of arcs of the spinal level, so efficient for its life that, even 

 when the brain is removed, the frog can execute under certain 

 stimuli a purposeful complicated movement such as that of trying 

 to wipe away with its foot an irritant drop of acid applied to its 

 head or back ; or, still more, if touched lightly between the scapulae, 

 will "lower its head at the first touch, and again more so at a 

 second, and at a third will, besides lowering the head, draw the 

 front half of its trunk slightly backwards ; at a fourth the same 

 movement with stronger retraction ; at a fifth give an ineffectual 

 sweep with its hind or fore-foot ; at a sixth a stronger sweep ; at 

 a seventh a feeble jump ; at an eighth a free jump, and so on." 

 Probably such an animal as the frog has all its reflexes congenitally 

 organized, whereas a dog, reaching the sensory level, has added 

 countless reflex-arcs to those inherited from its early ancestors of 

 the Insectivores which had long emerged from the spinal level, 

 retaining its old, perfecting its new inheritance, and eliminating 

 the unfit. Perhaps a faint picture of this long process may be 

 afforded by watching an experienced mountain guide ascending 

 an ice-slope with the aid of ice-axe, hand and foot. 



Integration of Raw Materials. 



Every group of animals in the higher ranks has its own entailed 

 property of innate reflexes, for example, the reflexes which subserve 

 the reflex functions of the cord : those of locomotion, muscular 

 and vascular tone, micturition, defsecation, impregnation and 

 parturition. These exist in an animal of the spinal level whether 

 or not it remains purely aquatic, partly aquatic, partly terrestrial, 

 arboreal or terrestrial. As the progressive groups ascend the ladder 

 of life they add to this inalienable heritage, gained we need not here 

 ask how, fresh reflex-arcs by response to new initial stimuli, forging 

 them by the incident of use. So, the original acquirements in the 

 past levels serve as starting points for raising the degree of their 



