n8 THE KINGDOM OF MAN 



incidence of gravity in plants which reacts on irritable 

 tissues, and is the explanation of the phenomena of 

 geotropism. These results have grown out of the 

 observations of Charles Darwin, followed by those of 

 F. Darwin, Haberlandt, and Nemec. 



A few words must be said here as to the progress of 

 our knowledge of cell-substance, and what used to be 

 called the protoplasm question. We do not now regard 

 protoplasm as a chemical expression, but, in accordance 

 with von Mohl's original use of the word, as a structure 

 which holds in its meshes many and very varied chemi- 

 cal bodies of great complexity. Within these twenty- 

 five years the * centrosome ' of the cell-protoplasm has 

 been discovered (see fig. 34), and a great deal has been 

 learnt as to the structure of the nucleus and its remark- 

 able stain-taking bands, the chromosomes. We now 

 know that these bands are of definite fixed number, 

 varying in different species of plants and animals, and 

 that they are halved in number in the reproductive 

 elements the spermatozoid and the ovum so that on 

 union of these two to form the fertilized ovum (the 

 parent cell of all the tissues), the proper specific number 

 is attained (see figs. 35 and 36). It has been pretty 

 clearly made out by cutting up large living cells 

 unicellular animals that the body of the cell alone, 

 without the nucleus, can do very little but move and 

 maintain for a time its chemical status. But it is the 

 nucleus which directs and determines all definite growth, 

 movement, secretion, and reproduction. The simple pro- 

 toplasm, deprived of its nucleus, cannot form a new 

 nucleus in fact, can do very little but exhibit irritability. 

 I am inclined to agree with those who hold that there 

 is not sufficient evidence that any organism exists at the 

 present time which has not both protoplasm and nucleus 



