GENETICS AND EVOLUTION — 659 
characters. Gates suggests they may be environmental, as for 
example the lowering of the temperature during periods of germ 
cell maturation. Others have suggested internal causes, such 
as the weakness of attraction between homologous chromosomes 
and the consequent failure of synapsis. 
The rate of mutations has been slightly increased in Drosophila 
by Muller through raising the temperature of cultures of this 
insect. In 1926 he showed that the X-ray had the power to alter 
and rearrange the genes. By exposing adult Drosophila flies to 
powerful X-rays he greatly accelerated the rate of the mutations, 
increasing these by up to 150 per cent. 
It is now thought probable by many geneticists that genes 
located in the chromosomes represent the sole vehicle of inherit- 
ance and that heritable variations can come only from changes 
in the genes. ‘These changes represent mutations. 
