444 V. THE NATURE OF EVOLUTION 



environment under which they are exposed, as in general do proteins, 

 the main component of the genes. Owing to this strong reversibility 

 a gene can return to its original structure when the environmental 

 effect which has caused the change is removed. 



The recovery of the original structure, however, is difficult, if a 

 profound change arises abruptly in a gene. The genes can recover their 

 original structure because the course leading to the original structure 

 is remembered, but the memory of the course may not be retained 

 clearly, if the change occurs too rapidly and extensively to remove the 

 structural factors which obstruct the progress of the change. In such 

 a case the change is inclined to be irreversible and called mutation. 



Mutation is uncommon, and usually the genes undergo extremely 

 gradual changes, which continue to proceed by degrees in directions 

 which are determined by the environmental factors. Such a change is 

 completely reversible, and can be regarded as the normal change of 

 the genes. 



The factors, which induce directly or indirectly the gradual, rever- 

 sible change, involve the stimulus coming from organs which are 

 continuously used excessively, as well as ordinary environmental factors 

 such as temperature, light, and diet. 



The change of a gene leads necessarily to a change of character 

 of the cell or of the organism which is governed by the gene. Organ- 

 isms may alter their characters when brought under a new environ- 

 mental factor. Since such a change is reversible, the acquired character 

 is lost w^hen the environment returns to the original state. 



However, if organisms are exposed always to a new environmental 

 factor without returning to the original environment, the previous 

 structure will be gradually forgotten with the fixation of the new 

 character responding to the new environmental factor. Thus acquired 

 characters will become fixed, not easily to be lost by the change of 

 the environment. 



Fossils forming an evolutionary series suggest that evolution 

 proceeds in definite directions, from the directing tendency of internal 

 structures, and this phenomenon is called orthogenesis. If a certain 

 environmental effect continues to exert its influence upon a gene or 

 genes, in producing a gradual change in the gene in a definite direc- 

 tion, the organism having the gene is to undergo orthogenesis. 



Bacteriophage can be looked upon as a free gene, and when acted 

 on by an inactivating agent such as formalin or antiserum, it tends to 

 change gradually in a definite direction until completely inactivated. 

 This change is usually reversible if the change does not proceed too 

 far and too rapid, the original character of phage being recovered on 

 the removal of inactivating agent, which can be compared to the 



