Many factors may be responsible for mutations. These are as follows:
(1) Radiations: X-rays and other ionizing radiations such as alpha, beta and gamma rays, neutrons, protons, etc. are found to induce mutations.
(2) Chemical mutagens: several chemicals are strongly mutagenic. Auerbach (1949) found that mustard gas acts as powerful mutagen in Drosophila. Formaldehyde and urethane when mixed with food on which Drosophila larvae grow and develop, produce mutations. Triazine dioxide, caffeine, phenol and some cancer producing compounds are also mutagens.
(3) Environment: high temperature acts as the most important mutagen under certain conditions. Muller and Timofeff Ressovsky (1935) have seen that in Drosophila mutation rates are doubled or trebled with every rise of 10 degree centigrade in temperature.
Rastogi,V. B.(1995): Organic Evolution