A PHYSICAL ANALOGY 121 



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 material 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 (i) that differ- 

 ences of the kind which are generally used to dis- 

 tinguish separate species may arise as single variations ; 



