HEREDITY. 25 



are all three present. If the supply of iron fails, it can be imagin- 

 ed, that finally some cells may be produced lacking the oxide. 

 If one of such cells happens to be a top cell, it may give rise to a 

 bud, a branch, visibly different from the rest of the plant, a 

 loss-mutation, bud-variation. 



Until now, the experiments tried to induce mutation by 

 external influences have been of a rather crude nature (Bla- 

 ringhem, McDougal). We cannot see, that there was any work- 

 ing-hypothesis underlying such experiments which induced 

 these authors to try particular injections, particular trauma- 

 tisms. We think, that the best chance for success in experi-- 

 ments of this kind, lies in subjecting plants to conditions, which 

 make it possible to regulate their available food-materials 

 (water-culture) and to substract necessary ingredients from 

 their food- medium for periods as long as is compatible with the 

 health of the plants. We would not be surprised, if plants under 

 such a treatment, produced buds, or branches, or seeds, lacking 

 in one or more genes. There is no reason to assume a fundament- 

 al difference between bud-mutation and germinal mutations. 

 It is evident that mutated branches give rise to mutated germ- 

 cells, recessive mutations are perhaps more frequently produc- 

 ed spontaneously as bud- mutation than as germinal mutations. 



Not only lack of an ingredient indispensable for a gene may 

 be thought of as the direct cause of a mutation, but as different 

 processes have different temperature-coefficients, it is not diffi- 

 cult to imagine, how, in abnormally high or abnormally low 

 temperatures, chemical processes, leading to the up-building 

 the supply of a certain gene, may be temporarily suspended. 

 We are here thinking of Tower's results with potatoe beetles. 



Even if we think it highly improbable, that we will ever wit- 

 ness a progressive mutation, a spontaneous acquisition of a 

 new gene by a higher plant or animal, we must remember, that 

 the acquisition of a new gene by a new species as the result of 

 cross-breeding need not be rare at all. Some authors seem to be 

 willing to accept the occurrence of progressive mutations, 

 wholly on the circumstantial evidence furnished by the exist- 



