ii4 THE MAIN CURRENTS OF ZOOLOGY 



Genetics, but owing to the great prominence of the 

 subject it is treated separately. Referring to Chap- 

 ter IV for a statement of Mendel's law of alternative 

 inheritance, we can say that it has been demon- 

 strated in poultry, guinea pigs, rabbits and other 

 animals. The fruit-fly which breeds rapidly and is 

 easily kept under observation, has proved an ex- 

 cellent subject for observations of the effects of 

 crossing and the study of unit characters. Morgan 

 and his students have made extensive contributions 

 to the study of heredity with this insect. They have 

 added much to support the conclusion that the 

 chromosomes are the normal mechanism for the 

 transmission of Mendelian characteristics. 



Experimental Morphology. The earlier experi- 

 ments of the last part of the nineteenth century were 

 carried on under the name of experimental mor- 

 phology experiments on animals with a view to 

 determine the changes undergone by changed condi- 

 tions of life. Placing eggs and developing organisms 

 under different conditions of environment and ob- 

 serving the results. The eggs and larvae of aquatic 

 forms are especially adapted for experiments of this 

 nature inasmuch as the chemical and thermal en- 

 vironment can be easily changed. The entire sur- 



