VII. THE MECHANISM OF ADAPTATION 389 



investigated. Bacteria can usually acquire the resistance against a 

 variety of chemical and physical agents, which exert some injurious 

 effects upon them, when cultivated under their influences. The 

 change is, as a rule, reversible, acquired characters being lost when 

 the influence is removed. 



Attention should be paid, however, to the fact that usually some 

 periods of time are required, after the removal of an injurious agent, 

 for the complete disappearance of the resistance, that is, for the 

 recovery of the former character, so that the change is often called 

 "Dauermodifikation" or durable modification. This shows that the 

 changed character is inherited through several generations although 

 meanwhile the character will become less and less distinct until at 

 last completely disappears. 



In higher organisms, acquired characters may appear not to be 

 transmitted to progeny, but since in the majority of cases the charac- 

 ters tend to continue for some periods after the romoval of the stimulus, 

 it must be admitted that the character is transmitted from the somatic 

 cells to other newly formed ones in the region. Accordingly, in this 

 respect, the adaptation of higher multicellular organisms should like- 

 wise be regarded as "Dauermodifikation" of somatic cells. On the other 

 hand, as the structural pattern of a cell is to be determined by the 

 genes contained in it, it should be considered that Dauermodifikation 

 is caused by a temporary change of the genes in the somatic cell. 



As already described, the genes have the property to alter their 

 structure responding to the environment or stimulus under which 

 they are placed, but as the change is reversible the removal of the 

 stimulus will lead to the recovery of the original structure. However, 

 a considerable period of time is usually required for the complete 

 recovery. Dauermodifikation is thus raised. 



3. Various Types of Adaptation 



As already mentioned, a new environment may act upon an 

 organism as a stimulus but when the organisms is exposed for a pro- 

 longed time to the same environment, the organism will complete the 

 change induced by the environmental factor, that is, it will acquire the 

 structure correponding to the new environment, which therefore comes 

 to have no influence upon the organism. The adaptation thus depends 

 upon the basic character of the protein itself. If the adapted struc- 

 ture, that is, structure which had completed the change induced by the 

 new environment was favourable for the organism, the organism would 

 be more fitted to continue its existence ; therefore, the adapted struc- 



