DISCONTINUOUS VARIATIONS 83 



a tree becomes "weeping," a genius is born. When a new 

 pattern of organisation or a new constitutional property turns 

 up, we may speak of a discontinuous qualitative variation. 



Historical Note. — The idea that organic changes might come 

 about by leaps and bounds is not novel, though the evidence 

 substantiating it is quite modern. 



Some of the older evolutionists, such as Etienne Geoffroy 

 St. Hilaire, believed in saltatory evolution, and were far from 

 agreeing with Lamarck that Nature is never brusque. 



Darwin also recognised that big steps may be taken suddenly — 

 e.g. in the origin of large-crested Polish fowls, black-shouldered 

 peacocks, short-legged Ancon sheep, but he thought that these 

 discontinuous variations occurred rarely, and would be liable to 

 be swamped by intercrossing. He relied rather on the action of 

 natural selection on the small, continuous variations which are 

 always forthcoming. 



But the modern appreciation of the importance and frequency 

 of discontinuous variations is mainly due to Bateson, who, in his 

 Materials for the Study of Variation (1894), gave many instances of 

 the sudden appearance of offspring which in some particular 

 diverge widely and abruptly from their parents ; and to De Vries, 

 who has observed the occurrence of " mutations " in many plants, 

 and has also followed them through generations, showing that they 

 tend to breed true ; and to Johannsen, who recognised the import- 

 ance of individual new departures in starting stable " pure lines." 



A Change of Yiew. — Darwin and orthodox Darwinians relied 

 in the main on the operation of selection on small individual 

 variations — many of which are nothing more than quantitative 

 fluctuations. If new adaptations and new discontinuous species 

 arise in this way, the small variations must be heritable, the new 

 character must be capable of cumulative increase by the per- 

 sistent outcrop of similar variations generation after generation, 

 the selection must be persistent and consistent, and a long time 

 must be allowed. 



