60 THE EVOLUTION OF IklAN. 



ontogenetic works are those of Kowalevsky, E. Ray Lan- 

 kester, and Eduard van Beneden, to which we shall present!}^ 

 again refer.^^ 



A more decided advance in general knowledge than was 

 effected by all these separate investigations, dates from the 

 year 1838, when the proof of the Cellular Theory suddenly 

 opened a new field of research in the History of Evolution. 

 The distinguished botanist, Schleiden, of Jena, having 

 proved by means of the microscope that every vegetable 

 body is composed of innumerable elementary parts, the so- 

 called cells, Theodor Schwann, of Berlin, a pupil of Johannes 

 Miiller, applied this discovery directly to the animal body.^^ 

 He showed, that not only in plants, but also in the bodies 

 of the most dissimilar animals, these same cells are dis- 

 tinguishable, under the microscope, in all the tissues, and 

 that they form the actual building material of organisms. 

 All the numerous tissues of the animal body, such as the 

 entirely dissimilar tissues of the nerves, muscles, bones, 

 outer skin, mucous skin, and of other similar parts, are 

 originally composed of cells; and the same is true of 

 all the various tissues of the vegetable body. These cells, 

 which we shall hereafter consider more closely, are inde- 

 pendent living beings, the citizens of the state, which con- 

 stitute the entire multi-cellular organism. The knowledge 

 of this most important fact was, of course, of direct service 

 to the History of Evolution also, in that it raised many 

 new questions, chiefly these : What relation have the cells 

 to the germ-layers ? Are the germ -layers already com- 

 posed of cells, and how are they related to the cells of the 

 tissues which afterwards appear ? What place does the 

 Qgg hold in the Cell Theory ? Is it itself a ceU, or is it 



