108 GENETICS 



accompanies sexual reproduction, constitute a pure 

 line or an unmixed strain. 



Third, in cases of asexual reproduction where the 

 progeny are simply the result of continued fission of 

 the original individual, a pure line may be said to 

 continue from generation to generation. 



In the second and third categories it should be 

 pointed out that the "pure line" is assured only so 

 long as asexual reproduction continues. It is quite 

 possible for an organism, heterozygotic in composi- 

 tion, to continue to breed true or to produce an ap- 

 parently puje line so long as asexual methods are 

 employed. As soon as such an organism, however, 

 changes to the sexual method of reproduction, seg- 

 regation of characters may occur and different 

 combinations result. 



6. Tower's Potato-beetles 



As an illustration of the effect of selection within 

 pure lines of the first category may be mentioned a 

 case given by Tower in his exhaustive experiments 

 on the Colorado potato-beetle Leptinotarsa decem- 

 lineata. Among the numerous cultures of this 

 beetle which were under control, a considerable 

 variation in color made its appearance. For con- 

 venience in classification these variations wxre 

 graded into arbitrary classes or graduated variants 

 (see p. 52) ranging from dark to light. 



When a male and a female from the extreme class 

 at the dark end of the series were allowed to breed 

 together, their progeny were not dark, but fluctuated 



