INTRODUCTION: THE FIELD AND PROBLEMS OF BIOLOGY 7 



which they called by various names and interpreted in various fashions. 

 By the early part of the nineteenth century these observations had 

 become more frequent, and came to include many kinds and parts of 

 plants, and a smaller number of animal tissues. No great interest or 

 importance was, however, attached to these findings. 



2. The Statement of a Hypothesis. In 1839 two men, the botanist 

 Schleiden and the zoologist Schwann, announced the hypothesis that 

 "all organized bodies are composed of essentially similar parts, namely, 

 of cells. ..." They came to this conclusion partly from the work of 

 others and partly from their own investigations. The}' had examined a 



Fig. 1.1. Robert Hooke's illustration of the cells in cork, from his Micrographia, published 

 in 1665. 



very large number of plant and animal parts in a special search for cellular 

 structure. Their generalization, however, went far beyond any actual 

 observations that they could have made or have gathered from the writ- 

 ings of others. Only a small fraction of the multitude of living things had 

 been examined for cells, and not all parts of even a single organism had 

 been thoroughly explored. Yet wherever Schleiden and Schwann had 

 made a careful microscopic examination they had found cells, and they 

 had also noted that cellular structure is often hard to detect and is demon- 

 strable only by careful technique. They had found cells in many or- 

 ganisms; their hypothesis was that all life was cellular in composition. 

 Both these men had many erroneous ideas about cells. They regarded 

 them as vesicles or spaces formed from a noncellular "mother liquor' 

 and thought that they were formed much as crystals are formed in 

 solutions. 



3. Corrections, Modifications, and Extensions of the Original Hypothesis. 

 The published hypothesis of Schleiden and Schwann stressed the impor- 



