THE EAW 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 

 sohd 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 ein 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 rolUng over to a position of equilibrium 

 on another face : this would be " discoijti4uoiig 

 yariation," or mutation, 



