34 THE MAIN CURRENTS OF ZOOLOGY 



These methods so obscure, and, at first, so difficult 

 to ferret out are now commonly demonstrated in 

 schools and training institutions. Many simple 

 experiments are now tried which give evidence of 

 the presence of these very minute germs in the air. 

 By exposing plates of sterilized gelatine in shallow 

 dishes with glass covers to keep out germs, in a room 

 before and after sweeping, in crowded school rooms 

 and in open country air, by allowing tap water to 

 trickle across a sterilized gelatine film, by allowing a 

 fly to walk upon it, etc. By these means the sterilized 

 plates become infected, they are immediately covered 

 and set away in a warm place. The growth of bac- 

 teria on the gelatine produces easily seen patches 

 and give evidence of the existence of these germs 

 which, individually, can be detected only by high 

 magnification. It is obvious that the rise of bac- 

 teriology with its special problems exerted wide 

 influence on all biological science and this justifies 

 classifying it with the outstanding advances of the 

 century. 



The Experimental Study of Heredity. The in- 

 vestigation of heredity is of surpassing interest. 

 There are so many theoretical and practical questions 

 involved. Perennial is the wish to know the way in 



