68 LECTUEES ON BIOLOGY 



the organic world into two separate kingdoms does not exist. 

 However much each highly organized animal may differ from 

 the higher plant, the history of their development proclaims 

 their common origin. Animals and plants are but two branches 

 of one tree of life. If, nevertheless, a line of demarcation is 

 still drawn, such a proceeding is justified only by considerations 

 of utility. 



The discovery and establishment of the modern cell-theory 

 is of comparatively recent date, for it is only since the begin- 

 ning of the past century that the work of Mathias Schleiden, 

 Schwann, v. Mohl, Max Schultze and others brought about a 

 complete revolution in the methods of scientific research and 

 made possible an undreamt-of advance in the whole of our 

 knowledge of Nature. 



As early as 1667 an English scientist, Robert Hooke, 

 succeeded in demonstrating by means of 'a microscope con- 

 structed by himself that a particle of cork is built up of an 

 immense number of single homogeneous parts. He it was, too, 

 who first used the name of cell in this connection, because of 

 the similarity of these forms with the cells of bees and wasps. 

 Not many years afterwards (1672) the English botanist Grew 

 published a comprehensive work (" Anatomy of Plants "), with 

 numerous microscopic pictures of sections of various parts 

 of plants, in which the cell-like structure may be distinctly 

 observed. Even then it was already perceived that the indi- 

 vidual ' bee-cells ' differed in structure and appearance, and 

 that homogeneous cells joined together and formed larger struc- 

 tures ; nor did these investigators fail to observe that in growing 

 parts the cells grew in size and multiplied in number. But 

 the true cause of all these phenomena, the seat of life in the 

 plant organisms, remained nevertheless hidden in darkness, for 

 what the early investigators saw and described was but a dead, 

 unimportant component, a secretion of the cell, its membrane. 

 Only with the discovery by v. Mohl and Schleiden of the living 

 contents, the cell-body, with the perception of the uniform 

 construction of the whole plant-world out of homogeneous ele- 

 mentary organs, and finally with Schwann's demonstration of 

 analogous conditions in animals, the cell-theory took the field 

 and gradually gained its present importance. 



