782 



SCIENCE. 



[N. S. Vol. XX. No. 519. 



esses form constituents of consciousness. 

 The words 'instinct' and 'will' do, how- 

 ever, not give us the variables by which 

 we can analyze or control the mechanism 

 of these actions. Scientific analysis has 

 shown that the motions of animals which 

 are directed towards a definite aim depend 

 upon a mechanism Avhich is essentially a 

 function of the symmetrical structure and 

 the symmetrical distribution of irritability. 

 Symmetrical points of the surface of an ani- 

 mal, as a rule, have the same irritability, 

 which means that, when stimulated equally, 

 they produce the same quantity of motion. 

 The points at the oral pole as a rule possess 

 a qualitatively different or greater irrita- 

 bility than those at the aboral pole. If 

 rays of light or current curves, or lines of 

 difiiusion or gravitation, start from one 

 point and strike an organism, which is 

 sensitive for the form of energy involved, 

 on one side only, the tension of the sym- 

 metrical muscles or contractile elements 

 does not remain the same on both sides of 

 the body, and a tendency for rotation will 

 result. This will continue until the sym- 

 metrical points of the animal are struck 

 equally. As soon as this occurs there is 

 no more reason why the animal should 

 deviate to the right or left from the direc- 

 tion of its plane or axis of symmetry. 

 These phenomena of automatic orientation 

 of animals in a field of energy have been 

 designated as tropisms. It has been pos- 

 sible to dissolve a series of mysterious in- 

 stincts into cases of simple tropisms. The 

 investigation of 'the various cases of tropism 

 has shown their great variety and there 

 can be no doubt that further researches 

 will increase the variety of tropisms and 

 tropism-like phenomena. I am inclined 

 to believe that we possess in the tropisms 

 and tropism-like mechanisms the independ- 

 ent variable of such functions as the in- 

 stinctive selection of food and similar 

 regulatory phenomena. 



As far as the mechanism of conscious- 

 ness is concerned no scientific fact has thus 

 far been found that promises an unravel- 

 ing of this mechanism in the near future. 

 It may be said, however, that at least the 

 nature of the biological problem here in- 

 volved can be stated. From a scientific 

 point of view we may say that what we 

 call consciousness is the function of a defi- 

 nite machine which we will call the ma- 

 chine of associative memory. Whatever 

 the nature of this machine in living beings 

 may be, it has an essential feature in com- 

 mon with the phonograph, namely, that it 

 is capable of reproducing impressions in 

 the same chronological order in which they 

 come to us. .Even simiiltaneous impres- 

 sions of a different physical character, such 

 as, for instance, optical and acoustical, 

 easily fuse in memory and form an insep- 

 arable complex. The mechanism upon 

 which associative memory depends seems to 

 be located, in higher vertebrates at least, 

 in the cerebral hemispheres, as the experi- 

 ments of Goltz have shown. The same 

 author has shown, moreover, that one of 

 the two hemispheres suffices for the effi- 

 ciency of this mechanism and for the full 

 action of conscioiisness. As far, however, 

 as the physical or chemical character of 

 the mechanism of memory is concerned, we 

 possess only a few starting points. We 

 know that the nerve cells are especially 

 rich in fatty constituents and Overton and 

 Hans Meyer have shown that substances 

 which are easily soluble in fat also act as 

 very powerful anesthetics, for instance, 

 chloroform, ether and alcohol, and so on. 

 It may be possible that the mechanism of 

 associative memory depends in some way 

 upon the constitiition or action of the fatty 

 compounds in our nerve cells. Another 

 fact which may prove of importance is the 

 observation made by Speck that if the par- 

 tial pressure of oxygen in the air falls be- 

 low one third of its normal value, mental 



