THE WORLD OF MICROSCOPIC LIFE 43 



widespread interest, and attracted other observers 

 to turn their attention to this newly-opened field. 



Knowledge of the world of minute organisms 

 progressed rapidly. Great improvements effected 

 in the compound microscope made it possible to 

 learn much concerning the structure of these small 

 living creatures and to bring into view forms of 

 life whose minute size had rendered them invisible 

 with the cruder instruments of the older observers. 

 Leeuwenhoek described the creatures he observed 

 as "animalcules," or little animals, and little ani- 

 mals many of them are; but many others turned 

 out to be minute plants. In these low forms, how- 

 ever, the plant and animal kingdoms draw nearer 

 together as if converging toward a common root, 

 and there are many forms about which it is very 

 difficult to decide to which kingdom they be- 

 long. 



A much greater insight into the nature and rela- 

 tionships of these forms followed the establishment 

 of the cell theory which was originally promulgated 

 in 1838 and 1839. According to this theory the 

 bodies of higher animals and plants are made up 

 of living units, the cells, which may be compared 

 to the bricks which are used in the construction of 

 a house. The small living forms studied by the 



