4 BREEDING 



ences between individuals, whether this has been 

 brought about by the stress of circumstance or the 

 skill of the breeder. In other words, the material 

 upon which selection, both natural and artificial, 

 has been supposed to operate has been assumed to 

 be these imperceptible differences, the occurrence of 

 which is referred to as the phenomenon of continuous 

 variation. 



This belief was attacked by Mr. Bateson in his 

 " Materials for the Study of Variation " in 1894, and 

 was severely shaken by the publication of Prof, de 

 Vries' '' Mutationstheorie " in 1901. The belief em- 

 bodied in both these works is that evolution has 

 not proceeded by the accumulation of the differ- 

 ences presented by continuous variation, but by the 

 differences which are furnished from time to time by 

 what is called " discontinuous variation " by Mr. 

 Bateson, and " mutation " by Prof, de Vries. 



The point at issue between the Selectionist, who 

 holds that evolution has been brought about by 

 the selection of continuous variations, and the Muta- 

 tionist, is this. It is admitted that species, as we 

 see them, constitute a discontinuous series ; they 

 are, with rare exceptions, distinct from and do not 

 merge into the species most closely allied to them. 

 The question is whether, when one species arises 

 from another, the new species arises at one bound, 

 i.e. in one generation, or whether it only comes 

 into existence after all the imperceptibly small 

 gradations, between it and the old species from 

 which it sprang, have been traversed over the 



