LECTURES IN BIOLOGICAL SCIENCES 



cal influences which produce ionization throughout the cell in- 

 crease the frequency with which mutations occur. It has grad- 

 ually become apparent, however, that the frequency of mutation, 

 both spontaneous and radiation induced, varies from one type of 

 cell to another, and from stage to stage in the development of 

 rapidly altering cells such as the gametes. It is clear then that 

 mutation is, either primarily or at least secondarily, dependent 

 on the metabolic condition of the cell. The interpretation of 

 th is is still very uncertain. Further, we now know a consider- 

 able range of chemical substances which can increase the fre- 

 quency of mutations. But again their exact mode of operation 

 is still not fully understood. In some cases, particularly in 

 bacteria, there is evidence that certain genes are more sensitive 

 to the action of particular mutagens than others. This, perhaps, 

 suggests that in these cases the mutations are brought about by 

 direct chemical interaction between the mutagen and the gene 

 itself, but in most cases it seems more probable that a number 

 of stages intervene between the action of the added chemical 

 and the final gene mutation. 



In evolutionary theory, the point of first importance is that 

 in all these cases of induced mutation the mutagenic stimulus 

 merely increases the frequency with which a wide range of mu- 

 tations occurs. It does not direct mutations into any particular 

 channels. These phenomena do indeed involve the production 

 of new hereditary variation under the influence of environmental 

 factors; but in the first place these factors are of very special 

 kinds, such as ionizing radiations or certain potent chemicals, 

 and in the second place the type of hereditary variation produced 

 has no precise relationship to the type of environmental influ- 

 ence which is operating. At the stage in the history of genetics 

 reached by the middle 1940's, there appeared to be very little 

 connection between the origin of new hereditary variation and 

 the directive action of the environment on the course of evolu- 

 tionary change. The situation was conventionally summed up 

 by saying that mutation was a "random" process. 



In more recent years the situation has already developed to 

 a certain extent. A few types of phenomena have been discov- 

 ered in which hereditary changes are produced which are not 

 random, but are rather precisely directed by the agent employed 



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