302 AN INTRODUCTION TO MODERN GENETICS 



setting up a cross in which a i : i segregation would be expected, and 

 then counting the divergence from this expectation which has been 

 brought about by differences in selective values. In these experiments, 

 however, the selection was usually only followed for one generation, 

 and only sufficed to measure the relative viabilities of two genotypes 

 under rather uimatural conditions. L'Heritier and Teissier^ have carried 

 on such cultures through many generations and followed the gradual 

 elimination of certain genes. They have also made some experiments 

 under natural conditions. Thus the common occurrence of wingless 

 species of inseas near the sea-shore has led to the suggestion that in 

 such situations the winged forms are at a selective disadvantage owing 

 to the danger of being carried out to sea by winds. L'Heritier, Neefs 

 and Teissier^ therefore released mixed cultures of winged and vestigial 

 (wingless) Drosophilae. They found, as predicted, that after some time 

 the winged forms were rarer than the vestigials. It is not perfectly clear 

 from their account that they had eliminated the possibiUty that the 

 winged forms had merely flown away elsewhere but the experiment is 

 mentioned here as an example of a tj^e of work which is of considerable 

 importance for the experimental study of evolution but which has been 

 surprisingly little taken up. 



9. The Inheritance of Acquired Characters^ 



The theory of the inheritance of acquired characters is primarily a 

 theory of the origin of hereditable characters, rather than a theory of 

 their preservation, as is the theory of natural selection. In fact, a beHef 

 in the inheritance of acquired characters can easily be combined with a 

 belief that all evolutionary advance is limited by natural selection. 

 Darwin at one time held both views. 



The inheritance of acquired characters may be taken to imply either 

 of two things: (i) that if an environmental agency affects the body of 

 an organism and produces a somatic change A^ in the course of genera- 

 tions A will become hereditarily fixed and inherited independently of 

 the environmental stimulus ; or (2) it may imply in addition that /i is a 

 change adapting the organism to the environmental conditions to which 

 it is subject. It is of course the second of these possibihties which gives 

 the theory its attractiveness and causes it to be tenaciously held in the 

 almost complete absence of evidence in its favour. This second form of 

 the theory is usually associated with the name of Lamarck. 



^ L'Heritier and Teissier 1937. - L'Heritier, Neefs, and Teissier 1930. 



3 Rev. Detlefsen 1925, Haldane 1932, Robson and Richards 1936. 



