THE SUMMARY OF PART V 447 



from use or disuse of an organ can be transmitted to the genes of 

 germ cells and will gradually be fixed in them to become inheritable. 

 The influence of the use of an organ upon genes appears to be so 

 intensive that it leads to a swift and extensive development of the 

 organ. 



The atrophy of an organ following disuse is similarly rapid, which 

 should, however, be naturally expected, since the memory of the pre- 

 vious structure must be strong and fresh if the development has 

 occurred swiftly by the use. Vestigial organs, remaining persistent 

 after a prolonged disuse, may therefore involve the structures whose 

 development was never raised by the use. Probably, vestigial organs 

 were produced aimlessly by an orthogenetic change of a gene not 

 directly involved in any use, and when they came to have a useful 

 value, the use was commenced leading to a rapid development. 



A gene is usually concerned with a restricted, peculiar structure 

 of protoplasm protein, but a change in a restricted portion of a protein 

 molecule cannot, as a rule, occur vv^ithout exerting any influence upon 

 other portions of the protein. Hence, an extensive development of a 

 certain character by the change of the corresponding gene is to be 

 associated more or less with developments of other related characters, 

 thus overspecialization of useless organs being sometimes established. 

 The production of primitive organs, which may be left as vestigial 

 organs, might also occur in this way. 



A gene can undergo a gradual change in a certain direction under 

 a given environmental factor, because it has the property to achieve 

 the change, that is, because it has the predisposition to undergo the 

 change. Such a predisposition may somewhat vary with the difference 

 in individuality of the genes. Individuals with genes having each parti- 

 cular predisposition may accordingly change the character in different 

 ways or at different rates even under the same environment, thus in- 

 dividual variation becoming more manifest. Out of them the fittest 

 will be selected by natural selection. Since the fittest thus selected 

 has the predisposition to proceed further in the change which will 

 produce the fitted character, the individual will continue to evolve in 

 an orthogenetic direction in so far as environmental factor persists. 



Newly acquired structure of a gene is unstable and may readily 

 return to the previous structure as the memory of the previous struc- 

 ture is fresh, and accordingly a new structure of a gene, produced by 

 a certain gradual change, may be put back readily to the previous 



