NEW TYPES AMONG ANIMALS 153 



new conditions die off, while those that are adapted persist and 

 give rise to new varieties. Here among the beetles we have the 

 same sort of 'changes as among the butterflies, but with greater 

 success in producing new and permanent varieties. 



Let us take now some other insects where the exact mechanism 

 of changes has been followed. The drosophila is a little fruit fly 

 about an eighth of an inch long. Everyone has seen it hovering 

 in swarms over decaying fruit. This little creature has been the 

 subject of some of the most careful biological studies ever made. 

 They have been carried on by Morgan with the help of his stu- 

 dents. For many generations the flies have been raised and their 

 pedigrees have been kept as carefully as those of race horses. In 

 one of the experiments on these flies Miss Hoge selected those 

 that had an unusual number of bristles on the sex-comb, which is 

 part of the leg. Normally the male, which alone has the comb, 

 has ten bristles in each comb. Miss Hoge selected those having 

 eleven or more in order to see whether she could gradually obtain 

 a strain having more than the normal number. During the fifth 

 generation of these many-bristled flies there suddenly appeared 

 various duplications of the legs that bore the combs. What 

 caused this mutation no one knows, and all attempts to produce 

 it in other animals have been in vain. 



When the abnormal flies were bred, their mutation was in- 

 herited. It did not always take the same form, but there was 

 some kind of a doubling of the limbs. When the flies lived at ordi- 

 nary temperatures the abnormality was slight and did not accord 

 with the Mendelian laws of inheritance which determine the pro- 

 portion in which the qualities of the parents will reappear in the 

 offspring. For example, to take a very simple case, if white peas 

 are pollenated from the flowers of red peas, a quarter of the next 

 generation will be white, a quarter red, and the remaining half 

 will be pink, showing the result of mixture. In the case before us 

 the matter is merely more complex, but the probable occurrence 

 of a given quality can be worked out in exact mathematical 



