THE SUMMARY OF PART V 445 



Stimulus coming from the use of organs or from a changed environ- 

 ment. In this case, phage is of course a gene-model. 



The gradual change of genes is completely reversible, but some 

 periods of time are necessary for the reversion to the original 

 structure after the removal of the changing factor, as the reversion 

 occurs also gradually. Consequently acquired characters, which have 

 not yet been fixed enough, usually remain for some periods after the 

 removal of the factor, a phenomenon known as "Dauermodifikation." 



Characters given rise to by mutation are generally injurious to the 

 organisms, whereas those given rise to by the gradual, reversible 

 change of genes are usually favourable. 



This may depend on that the reversible change, unlike mutation, 

 is induced by naturally existing environmental factors to which the 

 organisms have to be always exposed. If organisms could not endure 

 the change of usual envii'onmental factors, they would inevitably 

 become extinct ; as a natural result only the organisms capable of 

 bearing the change would be able to continue the existence. In order 

 to become the fittest, the organisms would have to acquire the faculty 

 not only to endure the change but to obtain a favourable character 

 through the change, so that the present-day organisms, which must 

 be the fittest, can attain characters adapted to the changed environ- 

 ments. 



The change of genes thus raised by the environmental change is 

 gradual and reversible, and leads to the production of favourable 

 character, whereas the abrupt action of some unnatural chemical or 

 physical agents upon genes may cause random changes ; thus muta- 

 tion may arise. The genes of an organism existing in the present day 

 must have structures most fitted to the existence of the organism, so 

 that the majority of random changes taking place in the genie struc- 

 tures would naturally result in the production of less fitted or 

 unfavourable characters. Thus, mutation is usually injurious. 



The adaptability of organisms to a new environment is based upon 

 the gradual change of genes producing the character suitable for the 

 new environment. The adaptability must have been developed because 

 it is an indispensable character for the organisms. The organisms 

 without this character would surely fail to continue their existence. 



Such a favourable character, however, is presumably not yet suf- 

 ficiently developed in extremely primitive organisms, but as they were 

 evolved higher, the mechanism of the gradual change of genes might 



