1)IS(,()Vi;hy 



of yolk in the egg, available for the embryo's first 

 l>criod of growth, it seizes upon what the head 

 iiid tail regions have not been able to profit by, and 

 luromcs not merely relatively, but also absolutely 

 bii^k'er. 



We thus see that a part possessing more active 

 metabolism has in normal circumstances a first call 

 upon the available food supply ; it is also, however, 

 more susceptible to harmful agencies, so that these, 

 acting in certain intensities, upset the relative powers 

 of the various parts to utilise food supply. 



Normally, the proper balance of any organism is 

 maintained by the interaction of its different parts, 

 struggling for food, stimulating or depressing one 

 another, embarking on overgrowth, or being reduced 

 by resorption. Indeed, the organism in a quite real 

 sense is the interaction of its parts, and in no sense 

 merely their sum. The form, the rate of growth, 

 the actions, the span of life itself — all are determined 

 by the interaction of the parts of the incredibly com- 

 plex self-regulating mechanism. 



The role of the dominant region in this flying play 

 of forces is of extraordinary importance. Again the 

 Planarians will illustrate the principle for us. These 

 flat-worms have a remarkable power of regeneration ; 

 the body may be chopped up into pieces, and each 

 piece may reorganise itself into a new whole. Each 

 piece may, but does not always do so : this depends 

 on the age and condition of the original animal, and 

 on the external environment. Now two things are 

 noticeable. If regeneration is to occur, the first part 

 to form will always be the head, the dominant region 

 which in normal development also is first laid down ; 

 and secondly, the organs of the piece will only be 

 reorganised if a head is formed. The most prominent 

 of these in the central region of the body is the 

 pharynx. 



If an animal is cut in two behind the pharynx, the 

 hinder half may form a head, or it may not. If it 

 does, a pharynx w'ill shortly afterwards appear in 

 its proper relative position ; if it does not, no pharynx 

 will arise. 



We may say that in animals capable of total re- 

 generation, like Planarians, the tissues when cut will 

 respond by forming a new dominant region ; but 

 when a dominant region has once been formed, they 

 react by forming the various organs of the body in 

 their due relation along the axial gradient. The 

 working of the living substance of the species is thus 

 altered under the influence of the controlling dominant 

 region; but when this is removed, as by a cut, any 

 region of it is capable of remodelling itself into a 

 fresh dominant region. 



This potential equivalence of all regions ceases to 

 exist in higher forms, but none the less the dominant 



region exerts a profound control over the rest of the 

 system. If, for instance, the limb of a young mammal 

 such as a rabbit is exposed to X-rays of a certain 

 intensity, it will develop into a small and stunted 

 thing. But if the head be exposed to X-rays, not 

 only will the head grow up stunted, but the whole 

 of the rest of the body, though shielded from the effect 

 of the X-rays, will be stunted too. The brain is the 

 " dominant region " of the mammal, and general 

 damage to it causes impairment of function in all the 

 members. 



Indeed, in the highest organisms, and more par- 

 ticularly in man, the whole creature may with pro- 

 priety be thought of as consisting of two main parts — 

 the conscious mind and the non-conscious nerve- 

 centres in the brain on the one hand, and on the 

 other the whole of the rest of the body, which is but 

 the brain's servant and box of tools. 



But even within the nervous system and within the 

 mind, the parts are built up into a hierarchy of 

 dominant and subordinate. 



It was Hughlings Jackson who first gave this idea 

 full weight, and in our own day Head especially has 

 upheld and illuminated it. 



The conscious centres of will and attention are 

 dominant over unconscious centres in the brain, and 

 these again over reflex centres in the spinal cord. 

 Not only this, but education and mental and moral 

 growth mean nothing else but a subordination of the 

 will and the attention to a changing series of ideas, 

 which rises (if the education is good, the mental develop- 

 ment a true one) in a scale from the simple to the 

 complex and general, from the instinctive to the 

 reasoned, from the selfish to the altruistic. Each of 

 these ideas represents a period of our life, a stage in 

 our mental development, and is associated with definite 

 past happenings, with actual events and moods and 

 thoughts and wishes. . . . 



" But what " (I can hear the reader say) — "what 

 is all this about ? WTiat connection has it with living 

 backwards ? 



Its bearing upon the problem is this. Reversal of 

 development led us on to resorption, resorption intro- 

 duced us to the idea of differential susceptibility, and 

 that again to systems built up as a hierarchy of 

 dominant and subordinate parts. 



But these, too, will show differential susceptibility ; 

 and so mav prove capable of reversal of dominance. 

 And, as a matter of fact, it comes about that even in 

 the most highly-organised and complex system of all, 

 in the system that underlies mind, such reversals of 

 development may happen. It is not often that 

 they happen ; but the phenomenon is now well known, 

 and goes by the name of " mental regression." Here 

 the patient, as a result of " shell-shock " or other 



