234 GENERAL BIOLOGY 



pomace fly, Drosophila, which Professor Morgan 

 has investigated with remarkable results. This 

 little fly is easily bred in captivity on fermenting 

 bananas, and as its life cycle is very short, it is 

 possible to obtain generation after generation the 

 year round, and in immense numbers. A number 

 of mutations have been obtained involving various 

 somatic characters, such as eye-color, length of 

 wings, etc., which breed true. In nearly every 

 instance, however, these characters are sex-limited : 

 that is, although transmitted through either sex, 

 they become manifest in only one. 



Economic Aspects of the Subject. --Previous 

 to the discovery of the laws of Mendelian inheritance, 

 animal and plant breeders were accustomed to choose 

 the types they desired and "breed to" them; in 

 other words, to practice strict artificial selection. 

 The fixing of the type in this way was a slow process 

 and its stability always problematical. The appli- 

 cation of the laws of Mendelian inheritance has 

 changed the procedure of the modern experimental 

 breeder and has substituted certainty for previous 

 uncertainty in the result, and at the same time greatly 

 shortened the time required to attain the desired 

 form. A concrete example will make this clearer than 

 any general discussion. ' Taken as a whole, English 

 wheats compare favorably with foreign ones in 

 respect of their cropping power. On the other hand, 

 they have two serious defects. They are liable to 

 suffer from the attacks of the fungus which causes 



