124 EVOLUTION 



it is correlated with another variation of 

 greater momentum or vital value. 



Another result of modern studies on varia- 

 tion requires to be stated very cautiously. 

 Evidence is accumulating to show that 

 organic structure may pass with seeming 

 abruptness from one position of equilibrium 

 to another. Changes of considerable amount 

 sometimes occur at a single leap. These 

 brusque changes are called " discontinuous 

 variations," or sometimes " sports," and, in 

 certain cases, " mutations." There is nothing 

 new in the suggestion that evolution may 

 sometimes have been by leaps and bounds, for 

 this was a favourite idea of Cuvier's evolu- 

 tionist contemporary and antagonist, Etienne 

 Geoffroy Saint-Hilaire; and it was also a pet 

 heresy of Huxley's. There is nothing new in 

 recognizing that discontinuous variations do 

 occur, for they correspond to Darwin's " single 

 variations " or " sports." What is new is that 

 we are beginning to accumulate facts in regard 

 to their frequency and their heritability. 



Sir Francis Galton compared organic struc- 

 ture to a polygonal model, so shaped as to 

 stand on any one of its sides. " The model 

 and the organic structure have the cardinal 

 fact in common, that if either is disturbed 

 without transgressing the range of its stability, 



