THE THEORY OF MUTATION 147 



takes place by natural selection acting upon individual 

 differences ; and in this case it is to be observed that 

 it is the struggle between individuals of the same species 

 which is of primary importance. On the mutation 

 theory it is only the competition between allied species 

 which interests us from the point of view of evolution. 

 Natural selection is thus regarded as having no influ- 

 ence in the formation of species themselves. On the 

 other hand, the gaps existing between genera and still 

 larger groups, such as families and classes, is still sup- 

 posed to be due to the destructive action of natural 

 selection determining the survival of the fittest species, 

 so that this principle is by no means ousted from its 

 prominent position in the philosophy of evolution even 

 as expounded by the mutationist. 



One further point. On the theory of mutation the 

 survival of useless structures becomes readily com- 

 prehensible. Indeed, a structure which is actually of 

 the nature of a handicap to its possessor may fail to 

 cause extinction if it is combined with a vigorous con- 

 stitution, or if it is correlated with other characteristics 

 which are sufficiently useful to make up for the dis- 

 advantages entailed. The survival of many apparently 

 useless and some apparently harmful structures is very 

 difficult to understand on the hypothesis of a con- 

 tinuous evolution by the survival of the fittest indi- 

 viduals. This is an argument upon which de Vries 

 lays considerable stress, although it may be pointed 

 out that it is usually very difficult to form a judgment 

 as to the real usefulness or otherwise of organs. 



102 



