56 GENETICS AND EUGENICS 



Variation of Animals and Plants under Domestication. 

 Bateson took up this work after Darwin's death and collected 

 a large number of facts concerning variation, which he at- 

 tempted to classify, but without great success. His results 

 are found in a book entitled Materials for the Study of Va- 

 riation, published in 1894. The most important conclusion 

 reached by Bateson, was one which Francis Galton had al- 

 ready stated with great clearness in 1889 {Natural Inheri- 

 tance), viz., that variations fall naturally into two classes, 

 continuous and discontinuous. Continuous variations are 

 those which are graded, the extremes being connected by a 

 complete series of intermediate conditions; discontinuous 

 variations are such as are separated by gaps in which no 

 intermediate stages occur. Bateson believed that discon- 

 tinuous variations are more important in species formation 

 than are continuous ones, because, where variations are 

 discontinuous, the action of natural selection is greatly sim- 

 plified. In discontinuous variation selection determines the 

 survival of one or the other of two distinct groups, since 

 intermediates do not occur and it is unnecessary to assign 

 selectional value to each plus or minus gradation of an organ. 

 Galton had earlier expressed the same idea, suggesting that 

 evolution may be like the behavior of a polyhedron when 

 pushed. If pushed or tipped a little, it returns to its former 

 position of equilibrium, merely oscillating back and forth on 

 the same face as before. But if it is pushed hard enough, it 

 rolls over on to a new face coming to rest in a new position 

 of equilibrium. Galton suggested that discontinuous varia- 

 tions may be species forming variations, stable from the start, 

 whereas slight or graded variations may have no lasting 

 effect, like the oscillations of the polyhedron on one and 

 the same face. This view was strongly supported a few 

 years later by the botanist De Vries in his theory of muta- 

 tion (1900-1903). 



Meanwhile variation was being studied from a new point 

 of view, which we may call biometry. Francis Galton (1889) 

 was the founder of biometry but its full development has 



