340 V. THE NATURE OF EVOLUTION 



antigenic substance is generally necessary ; this reaction may depend 

 upon the recovery of the former pattern which was provided and 

 strengthened by the repeated injection of the antigen, as the pattern 

 produced by the antigen can be regarded as the changed structure of 

 protoplasm protein of the antibody-producing cells. 



An elastic substance, as for example a rubber tube, may become 

 easily bent at a definite portion at which a bending was previously 

 applied. If the bending is repeatedly applied at the same portion, it 

 will become more supple, and the bend will occur at the portion even 

 when a force is applied at another portion. Anamnetic reaction is said 

 to occur likewise even when a proper substance different from the for- 

 mer antigen is injected. The structural reversibility of protoplasm pro- 

 tein can thus be analogized with the elasticity of elastic substances. 



On the other hand, there are many evidences to prove the presence 

 of elasticity in the protein molecule itself. Organisms are without 

 exception composed of protoplasm whose main components are proteins, 

 while protoplasm itself is elastic and accordingly all the organisms 

 are elastic, a fact which may be based upon the elastic nature of the 

 protein. Strong elasticity can actually be demonstrated in isolated 

 proteins such as gelatin or fibrin. As is well known, casein can pro- 

 duce an elastic paste if treated with alkali. Such an elasticity may 

 be attributed to the thread-like form of the molecules which can com- 

 bine end to end to form a long chain as in the case of other elastic 

 substances. The elasticity of proteins appears to be not only revealed 

 against physical forces but also against chemical agents. 



A splendid application of this elastic nature of proteins, i. e. the 

 reversibility, can be seen in the memory of nervous cells. When an 

 animal is repeatedly subjected to the same kind of stimulus, the re- 

 action to the stimulus will become more rapid and more exact, thus 

 resulting in the training effect and in the strong memory. The proto- 

 plasm structure of our brain cells may undergo a change in response 

 to a stimulus ; the change will soon fade away, but when afterwards 

 a related stimulus is given, the change will be revived owing to the 

 reversibility of the structure. This must be the recollection of the 

 stimulus. Every one knows that recollection is strong when the 

 stimulus was previously given repeatedly or severely, and the recol- 

 lection of a certain affair, when its "impression" was strong, is given 

 rise to by some stimulus which has no direct connection with the 

 affair. 



Some memories are long-lasting, strong impressions being generally 

 held in the brain cells for life. Since the protoplasm protein of the 

 brain is to be renewed repeatedly during such a long period of time, 

 the memory should be transmitted successively to newly formed pro- 



