THEORIES OF HEREDITY 3 



5. The basis of all known life is the cell. The typical cell 

 consists of a nucleus and a cell-body. It has been proved 

 beyond reasonable doubt that the nucleus, or rather a hypo- 

 thetical substance within it which has been termed the 

 " germ-plasm," is "the bearer of heredity." Some such 

 substance some substance which is the bearer of heredity 

 there must be of necessity. Cells multiply by dividing into 

 two or more daughter cells, the usual number being two. 

 The lowest plants and animals are single cells. Higher 

 organisms consist of two or more it may be billions of 

 cells united for the common good. The body of a multi- 

 cellular plant or animal is compounded of the cell-descendants 

 of a unicellular organism, which is usually a fertilized ovum. 

 Each cell of the body, though definitely related to its fellows, 

 is, in a very real sense, a distinct and separate living entity 

 a unicellular organism. Blood-cells and wandering connective 

 tissue-cells are free. Spermatozoa and ova are purely para- 

 sitic. Skin cells have been artificially transplanted. Had 

 we the requisite skill it is possible that we could transplant 

 every kind of cell. The death of the whole organism (somatic 

 death) is followed, only after a measurable interval, by the 

 death of its component cells. 



6. Watching the multiplication of a somewhat highly 

 organized unicellular animal (Stylonychia pustulata, an in- 

 fusoria^, Maupas observed that after two of these had con- 

 jugated, the resulting fertilized cell divided and re-divided 

 many times without conjugation occurring again ; but that if, 

 after a certain fairly definite number of cell-divisions, during 

 which millions of individuals had come into being, conjugation 

 did not occur again, the family ultimately died out. He found, 

 moreover, that the descendants of a conjugated pair did not 

 conjugate among themselves, but only with the members of 

 another family, the descendants of another conjugated pair. 1 



7. Now all this is the rule amongst higher plants and 

 animals. The ovum and the sperm are unicellular animals 

 of the same species. They differ in appearance, for the 

 large and passive ovum contains much nutriment, whilst the 

 small and active sperm carries an organ of locomotion ; but in 

 essentials they agree. Their nuclei are equivalent ; that is 



1 Arch. d. ZooL, 2 me serie, vii. 1889. Calkins, who has repeated 

 Maupas's experiments, disputes some of his conclusions. For example, he 

 claims to have proved that infusorians, if properly nourished, continue 

 to multiply indefinitely without conjugation. This point, as the reader 

 will perceive later, is not material so far as the present work is con- 

 cerned. 



