CHAPTER V 
THE THEORY OF MUTATION 
MotaTIon is the term applied by de Vries to express 
the process of origination of a new species, or of a new 
specific character, when this takes place by the dis- 
continuous method at a single step—a process which 
he regards as the most important if not the sole 
method by which new species or specific characters 
arise. We shall see that although de Vries has 
recently done much to forward the propagation of 
this idea, the belief that such a discontinuous process 
is the normal method by which new species come into 
existence has been developing for a considerable time. 
We have seen that those who accept the idea of 
evolution by the action of natural selection upon a 
series of minute and almost imperceptible variations 
are confronted with the difficulty of explaining how 
by this method there could arise a number of different 
structures or parts so co-ordinated as to share in a 
common function. Moreover, a closer examination 
of the actual processes of variation and inheritance 
render it doubtful whether the selection of continuous 
variations of even a simple characteristic can ever lead 
to the development of a permanent new race. The 
121 
