THE STUDY OF ZOOLOGY 13 



attempts to explain the phenomena of life in terms of chemistry 

 and physics. The rejection of the vitalistic hypothesis by 

 biologists generally must not be attributed to narrowness on 

 their part, but simply to the lack of satisfactory evidence to 

 support the vitalistic doctrine. Since the vital factor does not 

 exist, scientifically speaking, the scientist is compelled to do 

 without it, since he can deal only with those facts and factors 

 in the problems of protoplasm that are amenable to scientific 

 analysis and measurement. The main support of the mecha- 

 nistic interpretation, from a practical point of view, is the fact that 

 its application has actually overthrown hypothetical vitalistic 

 explanations of many phases of metabolism and replaced them 

 with understandable explanations in terms of chemistry and 

 physics. Such a view recognizes a common background for all 

 natural phenomena and assumes that living and lifeless objects 

 are forms of matter and energy. On this basis the organism is a 

 physicochemical system which owes its peculiar properties to its 

 physicochemical make-up, and which differs only in its internal 

 arrangement from known nonliving physicochemical systems. 



Cells. — Protoplasm exists in the form of small structural units 

 more or less distinctly marked off from one another, and known as 

 cells. In the Protozoa, the entire body of the animal is a single 

 cell; but in all higher forms, the Metazoa, the body is composed 

 of many cells, structurally and functionally differentiated. In 

 1838, Mathias J. Schleiden, a botanist, and Theodor Schwann, a 

 zoologist, each published observations on the structure of plant 

 and animal tissues which led to the formulation of what is known 

 as the cell theory. The central idea of this theory — that the 

 bodies of animals and plants are composed of cells and the 

 products of cells — was undoubtedly held by R. J. H. Dutrochet 

 as early as 1824, but a clear description of the cell nucleus by 

 Robert Brown in 1831 enabled both Schleiden and Schwann to 

 distinguish between cells and other structures, and led them to a 

 more accurate conception of the cellular structure of organisms. 

 Schwann in 1839 published a very precise account of cell struc- 

 ture in a variety of animals which surpassed in accuracy and 

 completeness that of any of his predecessors. The protoplasm 

 doctrine is a much later development and not until 1861 was it 

 clearly understood that cells are really masses of protoplasm and 

 that protoplasm is essentially similar in all organisms. This was 



