380 V. THE NATURE OF EVOLUTION 



lions induced in the fruit fly, Drosophila, by X-ray are mostly asso- 

 ciated with the production of lethal factors. 



Every existing organism must be the fittest or at least must have 

 the characters of a survival value and accordingly its genes m.ust have 

 the structures well fitted for survival. Therefore, the probability for 

 producing more fitted structures by a random change may be ex- 

 tremely small and so it will naturally be expected that the changed 

 structures are mostly deleterious far from being useful. This may 

 be the reason why mutation produces, as a rule, deleterious characters. 



However, strange to say, the gradual, reversible alteration of 

 the genes, as pointed out above, seems commonly to produce characters 

 favourable for the organism. This may depend upon that the revers- 

 ible change is induced by naturally existing factors such as tempera- 

 ture, light, and food, to the changes of which the organisms must 

 always be exposed. If organisms could not endure the change of such 

 •common environmental factors, they would surely become extinct, and 

 as a natural consequence only the organisms capable of enduring the 

 change would be able to continue the existence. 



As discussed in the previous chapter, the proteins including the 

 organisms composed of the proteins are provided with the property in 

 themselves to become adapted to the environmental factor under which 

 they are placed for long periods ; in other words, the protoplasm and 

 accordingly the gene can acquire the structure which is most stable 

 under an environment to which the organisms are exposed for prolonged 

 periods, so that the environment may become the most suitable one 

 to the organisms. Thus the changed environment which acted at the 

 beginning as a stimulus upon an organism becomes normal having no 

 influence upon the organism. If an organism achieves a change in 

 its character in such a way according to the alteration of an en- 

 vironmental factor, it may be said that an adaptation favourable for 

 the organism has took place. 



Naturally existing environmental factors may usually change grad- 

 ually, and so the protoplasm structure may also undergo the struc- 

 tural change responding to this environmental change gradually, thus 

 gradual, reversible change being established. If the structural pattern 

 thus induced by the gradual environmental change was unfavourable 

 for som.e organisms, the organisms would be perished ; the organisms 

 for which the newly produced pattern was favourable or at least unin- 

 jurious would be left as the fitted. This must be the reason why 

 reversible alteration of the gene always produces characters favourable 

 •or at least uninjurious to the organisms. 



On the other hand, the structural pattern induced by unusual 

 stimuli such as X-ray or some synthetic chemical agents, even if the 



