504 EVOLUTION, GENETICS, AND EUGENICS 



there were several other associated defects, and this was more or less 

 true of the results of Guyer and Smith, for not only were lenses defec- 

 tive, but the whole eye was often much smaller than normal and defec- 

 tive in other parts besides the lens. But it should be remembered that 

 Morgan and others make no claim that a gene change is confined to 

 one part of the body, and gene changes probably represent the limit of 

 possible specificity. It would not be strange, then, if a lens antibody 

 modified a certain gene that particularly affected the lens, but affected 

 other structures to a less extent. 



In closing the discussion about causes of new hereditary conditions, 

 we are forced to admit that at the present time we know practically 

 nothing as to causes. It seems highly probable that the environment 

 has had some controlling effect upon evolution, for changes in organ- 

 isms have run closely parallel with changes in climate in past geological 

 ages. The apparent effect may be due to natural selection, in that 

 radical changes in climate might merely eliminate most of the special- 

 ized types and open up the world for the plastic types to diversify and 

 by mutation to produce new adaptive forms. At the present time it 

 looks as though the heredity material, both through chromosomal 

 aberrations and through gene changes, is slowly differentiating inter- 

 nally through inherent forces and is very Uttle affected by the environ- 

 ment. This does not mean that the environment may not exercise 

 through selection an important guiding influence. It is still open to 

 question whether there may not be a long-time effect of somatic func- 

 tioning upon the germ plasm. In the very nature of the case it is 

 impossible either to prove or to disprove this possibility. 



MUTATION AND EVOLUTION 

 T. H. MORGAN' 



What bearing has the appearance of these new types of Drosophila 

 on the theory of evolution may be asked. The objection has been 

 raised in fact that in the breeding work with Drosophila we are dealing 

 with artificial and unnatural conditions. It has been more than im- 

 plied that the results obtained from the breeding pen, the seed pan, the 

 flower pot and the milk bottle [used as breeding-container for Droso- 

 phila] do not apply to evolution in the "open," nature "at large" or 

 to "wild" types. To be consistent, this same objection should be 

 extended to the use of the spectroscope in the study of the evolution 



' From A Crilique of the Theory of Evolution. Princeton University Press, i g 1 6. 



