INTRODUCTION 



15 



in insects, or who discovered the small one-celled animals in pond water. 

 No one of them stands out preeminently as a representative of this 

 movement. It will be sufficient to point out that the steady improvement 

 of the microscope in the two centuries and more since the time of Mal- 

 pighi and his contemporaries has made it possible to study more and more 

 minute structures. Progress in anatomy, in so far as it concerns the 

 smallest structures, has at all times depended on, and kept pace with, 

 the improvement of the microscope. The physiologist must often watch 

 his experiment with the aid of a good microscope. Classification of 

 animals and plants in many cases shifted to an entirely new basis 



Fig. 6. — Antony van Leeuwenhoek, 1632-1723. {From GarrisorCs History of Medicine.) 



when observation of minute structures became possible. Microscopic 

 study of fossils reveals features of importance. Naturally, each of these 

 subjects has grown as the microscope grew. From an instrument Kke 

 that illustrated in Fig. 4, which is by no means the crudest one known, 

 the microscope evolved step by step to the splendid optical aids of the 

 present time. Today the biologist uses microscopes fitted with oil 

 immersion objectives and condensers for increasing the illumination, 

 lenses corrected for chromatic and spherical aberration, devices for 

 delicate adjustments of focus, and other refinements. Whether further 

 striking improvements are still possible is uncertain. 



Classification. — During all these centuries of observation, in the 

 early Greek period and in the period following Vesalius, many kinds of 



