V. GRADUAL ALTERATION OF THE GENE 381 



pattern is induced gradually and accordingly is reversible, must be 

 unfamiliar to the organism and the organisms are not accustomed to 

 the pattern ; and since, as above pointed out probability of a pattern 

 produced at random of becoming favourable for the organism must be 

 extremely small, patterns produced at random by unnatural factors 

 must, as a rule, be injurious to the organisms. In addition, unnatural 

 factors are usually applied suddenly in experimental researches, the 

 changes are generally irreversible, resulting in mutation. 



