HEREDITY • 341 



Mutations. Although it has not been possible to cause new 

 genetic characters to appear by the application of some environ- 

 mental change, new characters do appear in animal and plant 

 populations. These are called mutations and their cause is at pres- 

 ent unknown. Not all varieties of animals or plants are alike in this 

 respect; in some, mutations occur with considerable frequency; in 

 others their occurrence is rare. In the fruit fly, Drosophila melano- 

 gaster, which has served as excellent material for the study of 

 inheritance, mutations have occurred with some frequency in stocks 

 kept in the laboratory for experimental purposes. Other species of 

 this same genus appear to be much more stable and mutate but 

 rarely. 



The facts that are known concerning the causes of origin of new 

 characters are few indeed. Mention has been made of the fact that 

 sometimes in the maturation of the germ cells the members of a 

 synaptic pair of chromosomes do not sever equally, thus leaving an 

 excess of chromatin on one and a defect on the other. This results 

 in somatic effects that are inheritable. It is also known that certain 

 environmental conditions, for example high temperature, affect the 

 process of synapsis and result in changes in the cross-over of genes 

 located on the same chromosome. The most clear-cut effect of 

 environment on germinal constitution is the recently demonstrated 

 fact that when a race of Drosophila (the fruit fly) is exposed to 

 radioactive substances, mutations occur distinctly more frequently 

 than in races that have not been exposed. Such changes may be 

 referred to some unknown physico-chemical action of the ray on 

 the genes. Conjectures have been made that the effect of radio- 

 activity may account for the origin of many new characters in 

 animals and plants in general. But no conclusions can be drawn at 

 present. The nature of the relations of the chromosomes during 

 synapsis affords an opportunity for a wide variety of recombinations 

 that may account for genetic changes which appear from time 

 to time. 



