1.— PHYSIOLOGV. 217 



muscle or gland, the other has been hitherto in no way related to such a 

 result ; after many repetitions of the association of these events it is 

 found that that one which previously had never resulted in this particular 

 activity, comes to have this result as certainly as the other. 



The sight and smell of food in any hungry animal results in the secretion 

 of saliva because the cells to which the effect of these visual and olfactory 

 stimuli is referred are anatomically connected with cells that set the 

 salivary gland in action ; the cells on which some particular sound takes 

 effect are not anatomically connected with them, and this particular sound 

 Las therefore no effect upon them. But with the establishment of the 

 conditioned reflex the anatomical connection comes into existence. As a 

 result of a functional reaction of nerve-cells to disturbances in other nerve- 

 cells with which they were not previously anatomically connecteil, a 

 structure appears which is indistinguishable so long as it lasts from the 

 structures that constitute any other reflex arc. The conditions that 

 determine its persistence or effacement have been, and are being, studied 

 as thoroughly as were those which allow it to appear. The outcome of 

 these studies must be of incalculable importance in evolutional physiology. 

 They are being watched with the keenest interest doubtless by all 

 biologists, but more especially by those who believe that physiology has 

 to take a much bigger part in the solution of some of the fundamental 

 difficulties of biological science than it has been able to take in the jmst. 



But if and when it is possible to trace the origin of structures to 

 functional reactions of cells, and to reactions that depend upon the 

 chemical properties of the cell substance ; and if and when this is possible 

 not only in the connective tissues, but also in the nervous system, the 

 functions of which have so controlling an influence on the operation of 

 every part of the body ; until it becomes clear that the results of changes 

 in such influence reappear in succeeding generations, the study of functions 

 can have no bearing upon the ultimate problem of biology, the evolutional 

 history of life upon the earth. Pavlov communicated to the last Inter- 

 national Congress of Physiology in 1923 some results of experiments that 

 he had done upon this subject which, when confirmed, would electrify the 

 atmosphere. Conditioned reflexes that are established only after many — 

 eighty or a hundred — repetitions of the associated stimulus, in each suc- 

 ceeding generation require fewer and fewer rej)etitions, and in the fourth 

 may be established after only four. In April of this year he wrote to say 

 that owing to other work he had not been able to give the necessary time 

 to confirmation of these results. We are content to wait. 



In the great question whether characteristics developed in the life of 

 an individual have any influence on descendants, experimental evidence 

 must come slowly. In what is called parallel induction a step has been 

 taken which is probably of greater importance than is generally conceded. 

 External influences that affect the bodily characteristics of an organism 

 affect also the germ-plasm in such a way that these characteristics appear 

 in the first, and even, in a less degree, in the second generation, born after 

 the external influences have ceased to operate. While such experiments 

 furnish evidence only of a temporary change in the properties of the 

 germ-plasm, one that may be put down to the lodgment in it of uuassimi- 

 lated foreign matter that is gradually eliminated, the fact that the eternal 



