CHAPTER IX 

 EVOLUTION AND MUTATION 



1. The Significance of Mutation 



As discussed in Chapter V, environmental effects may give rise in 

 ■organisms to adaptive changes, which are usually advantageous, as a 

 result of the gradual, structural changes of the genes. The environ- 

 m.ental effect is gradually transferred to genes through the cytoplasm 

 or hormones. Such a change of genes is reversible, so that the change 

 will disappear when the environmental condition is returned to the 

 original state. 



On the other hand, occasionally, strong physical or chemical agents 

 can act directly upon the genes, producing sudden, irreversible changes. 

 A change belonging to this category is called mutation. It is usually 

 deleterious to the organisms in contrast to the gradual, reversible 

 change. Up to the present, this latter change has been disregarded 

 presumably because of its reversible nature, whereas the former, irre- 

 versible change, namely mutation, has attracted universal attentions 

 and is generally regarded as one of the main causes of evolution. 



However, such a random change can by no means be considered to 

 be significant for the evolution, as was already emphasized by Osborn. 

 In fact, as just mentioned, characters raised by mutation are mostly 

 deleterious to the organism far from being useful. 



The mechanism, by which the abrupt change of the gene is not 

 only prevented but on the corttrary favourable gradual change is induced 

 in the gene, must be of the utmost importance for the establishment 

 of the evolution, and consequently such a mechanism might have been 

 considerably developed even when the organisms were in rather 

 primitive stages. It should be remembered that the adaptive, rever- 

 sible changes are common even in the primitive organisms, such as 

 bacteria and protozoa, whilst mutation is rather unusual. In higher 

 animals, environmental effects may be modified and unspecialized by 

 hormones, thus abrupt changes being completely prevented. But, at 

 the same time, the reaction modus of higher organisms against com- 

 mon environmental factors may become restricted, and the dii^ection of 

 the evolution is accordingly confined in a narrow scope. 



It is believed that Australia and New Zealand have been separated 



