58 THE PROBLEMS OF EVOLUTION 



by virtue of environmental association estab- 

 lished through the cytoplasm. Even mutations, 

 therefore, must be a product of some phase of the 

 environment acting directly or indirectly on the 

 chromosomes. This becomes increasingly evident 

 in the light of experiments with X-rays, notably 

 the recent researches of Muller ^ on Drosophila, and 

 the discoveries of Babcock and Collins ^ in Cali- 

 fornia concerning the effects of terrestrial radiation 

 on the same insects. The body is a complex phase 

 of the environment, far more complex, indeed, 

 than some of the external conditions which give 

 pronounced results in the individual, and it is 

 quite possible that it may undergo changes from 

 time to time which produce mutations. To what 

 extent such changes are associated with fluctua- 

 tions of the external environment it is impossible 

 to say, but it is at least probable, as we have noted, 

 that independent changes are not likely to modify 

 the heritage. The changes due to increasing age, 

 for example, cannot be referred to external en- 

 vironmental stimuli, save only to the extent that 

 everything in the body is a product partly of these 

 stimuli, directly or indirectly. We have no reason 

 to believe that such changes affect the offspring of 

 an individual in fundamental ways. 



The importance of these various changes to the 



f Science N. S., Vol. LXVII, p. 82, 1928. 

 ^ Science Supplement, p. x, Aug. 2, 1929. 



