THE RAW MATERIALS OF EVOLUTION 105 



general result of the modern study of variability 

 is the evidence that changes of considerable 

 amount sometimes occur at a single leap. 

 These brusque changes are called "discontinuous 

 variations/' and, in certain cases, "mutations." 

 Lamarck said, "Nature is never brusque," and 

 we usually look askance at reports of " Jack-in- 

 the-Box " phenomena in nature ; but, through the 

 solid work of Bateson, De Vries, and others, there 

 is more reason to-day than there was fifty years 

 ago to believe that organic structure may pass 

 with seeming abruptness from one position of 

 organic equilibrium to another. 



DISCONTINUOUS VARIATIONS. In the individual 

 development of an embryo there is gradual con- 

 tinuous change from hour to hour, from day to day ; 

 and if we suppose similar changes to occur, not as 

 normal stages in the development of one creature, 

 but as new steps of progress in successive genera- 

 tions of creatures, we have the individual variations 

 that Darwin most believed in as furnishing the 

 raw materials of evolution. But in many a develop- 

 ment, such as that of a starfish or a butterfly, 

 there is in a certain sense discontinuity ; there is a 

 crisis, when the developing creature recommences 

 on a new track ; and this sort of change occurring, 

 not as a normal event in individual development, 

 but as a new departure in racial evolution, would 

 be a " discontinuous variation." Using Galton's 

 simile, we can picture a polyhedron oscillating or 

 rocking on one of its faces: this would be an 

 " individual variation," or fluctuation ; we can also 

 picture it rolling over to a position of equilibrium 

 on another face : this would be " discontinuous 

 variation/' or mutation r 



