THE THEORY OF MUTATION 157 
According to the view upheld by Wallace, Weismann, 
and others, the actual origin of specific distinctions 
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, are 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 
when the latter is 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 either combined with a vigorous 
constitution, or 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 
