58 EVOLUTION OF LIVING ORGANISMS. 



not yet know what causes them to appear. Darwin be- 

 lieved that great and sudden changes in the environment. 

 increased variability, a conclusion which is probably well 

 founded, but is difficult to prove, since we know so little 

 about the range of variability in nature under usual and 

 unusual conditions. Some recent experiments seem to 

 have clearly established the most interesting fact that 

 mutations can be brought about by altering certain 

 stimuli in the environment. But it must not be for- 

 gotten that the experimental conditions may only have 

 favoured the survival of pre-existing mutations, and not 

 actually caused their first appearance. The American 

 investigator Tower, experimenting on the Colorado 

 beetle (Leptinotarsa), found that on exposing some in- 

 dividuals at a certain stage and for a certain time to 

 extremes of heat and dryness there appeared in their 

 offspring beetles differing remarkably from the parents 

 in colour and pattern. That these new forms were real 

 mutants, and not modifications, was proved by their 

 breeding true under normal conditions, and when crossed 

 with the parent form giving the proper mendelian pro- 

 portion of parent and new types in the second generation. 

 Similar results have been obtained by Morgan with the 

 fly Drosophila, and by others with plants. For factorial 

 changes of this kind to produce lasting results, constant 

 inheritable mutations, they must of course be persistent. 

 It is a mistake to confuse, as is often done, such changes 

 brought about by the direct effect of external or internal 

 stimuli on the germ-plasm with alleged cases of the so- 

 called inheritance of acquired characters. 



These artificial mutations are ail of the retrogressive 

 kind, and the mutants, the new forms, are all recessives. 

 So that the result of the experiment is the loss or 

 suppression of one or more of the factors of inheritance. 

 But we may look forward to the possibility of being able 

 some day to produce progressive mutations in any desired 

 direction j a discovery of such immense practical value 



