THE SCOPE AND METHODS OF GENETICS 289 
to heredity; (f) theories as to the mechanism that brings about the 
observed regularity in heredity, including theories of linkage, cross- 
overs, and other phases of neo-Mendelian heredity. 
THE IMPORTANCE OF THE CELL THEORY IN GENETICS 
All organisms are composed of minute units called cells. A cell 
is the smallest particle of living matter capable of growing and multi- 
plying. The only continuity between two successive generations is a 
cellular continuity, involving a continuous series of cell divisions. 
All cells are the offspring by division of previous cells. We distinguish 
two main categories of cells: body or somatic cells and germ cells. 
In germinal reproduction, the only kind of reproduction possible 
in the more highly differential animals, the material continuity 
between parent and offspring is through the germ cell. A parental 
germ cell produces an offspring. The germ cell therefore is called the 
hereditary bridge. 
The only way, then, in which a parent can transmit his or her 
characteristics to an offspring is through the germ cell. Every 
hereditary character, whether old or new, must have its differential 
cause in the germ cell. The germ cells are therefore called the bearers 
of the heritage, and in the next chapter Professor M. F. Guyer gives 
an account of the cellular basis of variation and heredity. 
