GROWTH OF BIOLOGY m THE NINETEENTH CENTURY. 463 



and physiology have received a solid foundation in the theory of cells 

 and of protoplasm, just as chemistry has in the doctrine of atoms and 

 molecules. 



A series of very important ideas has arisen with the cell theory. If 

 plants and animals represent in a certain way colonies or states of 

 socially connected elementary living beings, vital processes are nothing 

 more than highly complicated resultants of numerous elementary proc- 

 esses which are performed in the cells. Thus it was suggested to draw 

 parallels and to institute instructive comparisons, on the one hand, 

 between the individual members of a human state and the adjustments 

 which a state necessitates, and, on the other hand, between the structure 

 and the life of the vegetable and animal body. The law of the division 

 of labor and of differentiation, which in human society causes separa- 

 tion into special professional classes and the immense diversity of social 

 employments, was rightly, adduced by Milne-Edwards, by Spencer, 

 and by many others to illustrate the building up of the vegetable and 

 animal body from its organs and tissues. With Lionel Beale and Max 

 Schultze we learned to distinguish in histology between a formative 

 substance, the protoplasm of the cells, and the product of their forma- 

 tion or work, and recognized that the various cells as the}^ assumed in 

 the service of the whole organism different functions or work, accord- 

 ing to time and place and their relations to one another, became corre- 

 spondingh^ diversified in their intimate structure, and that in this way 

 the various tissues and organs came into existence. 



The scientific elaboration of the theory of cells and tissues has occu- 

 pied many naturalists for several generations, and they have erected 

 the stately palace of the microscopic anatom}' of plants and animals. 

 Yet even here man}- important questions await solution, especially that 

 of the finer structure of the cell-nucleus and of protoplasm, and the 

 question of the microscopic structure of the nervous system and of 

 the organs of sense, concerning which almost every 3'ear still brings 

 us new investigations and new discoveries, sometimes of great 

 importance. 



With the aid of the compound microscope biological research has 

 in the passing century opened to our inspection a second new and sov- 

 ereign world of life, the world of the simplest unicellular organisms, 

 which were introduced into classification by many investigators as 

 an intermediate kingdom between plants and animals— the protista. 

 Great was the wonder, in the middle of our century, when Ehrenberg 

 discovered that whole geological formations originate from (erdschich- 

 ten) very minute organisms, often hardly visible to the naked eye, 

 which grow in fresh water and in the sea in immense numbers. For 

 when, at death, their soft protoplasmal bodies decompose, their hard 

 shells and skeletons of carbonate of lime or of silica still remain, and 

 sinking by their weight to the bottom produce, in thousands of years, 

 SM 1900 33 



