104 BIOLOGY AND ITS MAKERS 



and also upon the minute Crustacea commonly called water- 

 fleas, which are just large enough to be distinguished by the 

 unaided eye. We should remember also that in the same 

 period the microscopic structure of plants began to be inves- 

 tigated, notably by Grew, Malpighi, and Leeuwenhoek (see 

 Chapter IV). 



In addition to those essays into minute anatomy, in which 

 scalpel and scissors were employed, an endeavor of more 

 subtle difficulty made its appeal; there were forms of animal 

 life of still smaller size and simpler organization that began 

 to engage the attention of microscopists. A brief account of 

 the discovery and subsequent observation of these micro- 

 scopic animalcula will now occupy our attention. 



The Discovery of the Simplest Animals and the Prog- 

 ress of Observations upon Them 



These single-celled animals, since 1845 called protozoa, 

 have become of unusual interest to biologists, because in them 

 the processes of life are reduced to their simplest expression. 

 The vital activities taking place in the bodies of higher animals 

 are too complicated and too intricately mixed to admit of 

 clear analysis, and, long ago, physiologists learned that the 

 quest for explanations of living activities lay along the line 

 of investigating them in their most rudimentary expression. 

 The practical recognition of this is seen in our recent text- 

 books upon human physiology, which commonly begin with 

 discussions of the life of these simplest organisms. That 

 greatest of all text-books on general physiology, written by 

 Max Verworn, is devoted largely to experimental studies 

 upon these simple organisms as containing the key to the 

 similar activities (carried on in a higher degree) in higher 

 animals. This group of animals is so important as a field 

 of experimental observation that a brief account of their 



