TO4 BIOLOGY AND ITS MAKERS 
and also upon the minute crustacea commonly called water- 
fleas, which are just large cnough 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 (sec 
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 
