THE FOUNDATIONS OF GENETICS 



269 



tion some of the different qualities of the first. A guinea pig may 

 have the black color of its father and the angora coat of its mother. 

 Such variations are based on the rearrangement of heritable 

 components and are consequently heritable. They are an impor- 

 tant source of change in organisms, but are necessarily Umited 

 by the range of characters present in the species. They have 

 been widely exemplified in the development of domestic races. 



Mutations. When an individual appears with characters dis- 

 tinctly different from those of preceding generations it is said 

 to be a sport if its offspring return to the parental condition. 

 In some cases, however, differences which appear thus suddenly 

 are heritable and constitute permanent characters of the succeeding 

 generations. The 

 characters in ques- 

 tion are then said to 

 be mutations, and 

 the individual pos- 

 sessing them is a 

 mutant (Fig. 155). 

 Such variations are 

 due to an abrupt 

 qualitative change in 

 the organism and 

 because of their her- 

 itability they play an 

 important part in the 

 modification of 

 species. Their value 

 as a cause of evolu- 

 tion is treated else- 

 where in this volume. 



Source of Varia- 

 tions. Walter sum- 

 marizes this question 



with four opinions: (1) Darwin considered variations as axiomatic; 

 (2) Lamarck and his followers looked upon them as either produced 

 by the environment or by the organism in response to environ- 

 mental conditions; (3) Weismann regarded them as purely an 

 intrinsic product of the body; (4) Bateson regards incjuiry into 

 the causes of variation as premature. 



a' 



b' 



Fig. 155. — A mutation in Drosophila melanogaster 

 compared with the normal character, a, a', normal 

 eyes; b, b', bar eyes. (From Morgan et al., 

 Mechanism of Mendelian Heredity, with the per- 

 mission of Henry Holt and Company.) 



