128 THE THEORY OF MUTATION 
complete rearrangement of the remaining corpuscles 
in order to arrive at a new position of equilibrium, 
and this change would be accompanied by a marked 
alteration in the chemical properties of the atom itself. 
In like manner the chemical composition of the living 
substance of a race of organisms may be conceived to 
alter step by step, every such step being accompanied 
by comparatively unimportant changes in its visible 
characters, until the time arrives when any further 
alteration must be associated with a deep-seated revo- 
lution in the constitution of the living substance, and 
with a corresponding marked mutation in the external 
features of the members of the race. 
The first really definite attempt to collect and co- 
ordinate the facts of discontinuous variation was made 
by Bateson in his book entitled ‘Materials for the 
Study of Variation,’ published in 1894. The intro- 
duction and concluding remarks at least of this volume 
ought to be read by everyone who is interested in 
these subjects. The bulk of the book contains a mass 
of materia] of great value to specialists. 
After pointing out the difficulties which prevent his 
acceptance of the orthodox belief in the origin of dis- 
continuous and apparently adaptative types of animals 
and plants through the action of natural selection on 
minute variations, difficulties to which we have 
already paid some attention, Bateson records his con- 
viction that the facts of discontinuous variation afford 
a way out of the difficulty. He shows (rz) that differ- 
ences of the kind which are generally used to dis- 
tinguish separate species may arise as single variations ; 
