102 The Science of Life. 



which would be an impertinence, we cannot deny that 

 it was long before their work led to any new general 

 idea ; all that can be said is that they revealed a new 

 world of detail which both physiologist and embryolo- 

 gist had to take account of, and which in a few cases 

 helped to deepen physiological and embryological un- 

 derstanding. 



The first generalization of importance was within the 

 nineteenth century, namely Bichat's analysis of the 

 organism into a series of tissues with definite structural 

 characters nervous, muscular, connective, glandular, 

 &c. We now define a tissue as an aggregate of more 

 or less uniform cells or modifications of cells, but this 

 definition implies a step of analysis beyond Bichat's. 

 The step he took was really this the anatomist had 

 disclosed organs, such as heart and lungs; Bichat 

 analysed these organs into their component tissues 

 (muscular, connective, nervous, &c.), and also en- 

 deavoured to show that the function of the organ was 

 expressible in terms of the properties of these tissues. 



Very gradually, by numerous isolated observations, 

 an approach was made towards laying that foundation- 

 The Ceil- stone of modern biology which is usually 

 theory. spoken of as the cell-theory. 



In 1838 Schleiden showed that plants were built up 

 of cells and modifications of cells, and discovered the 

 origin of the plant-embryo to be a single cell or ovum. 

 In the following year Schwann extended these two 

 observations to animals, and thus the "cell-theory" 

 was formulated. " No other biological generalization," 

 says Prof. Wilson, "save only the theory of organic 

 evolution, has brought so many apparently diverse 

 phenomena under a common point of view, or has 

 accomplished more for the unification of knowledge." 



The cell-doctrine in its full statement includes three 

 propositions: the first morphological, the second em- 

 bryological, the third physiological. 



(i) Morphological. All organisms are either single 

 corpuscles of living matter (the unicellular Protozoa, 

 Protophytes, and Protists) or are built up of a large 

 number of such corpuscles and modifications of these 



