i2 4 THEORIES OF HEREDITY 



The fact that each germ-cell, placed under appropriate 

 conditions, will develop somatic cells like those of the 

 parent, is explained by the supposition that all the latter 

 cells give off gemmules, which are stored up in the germ- 

 cells. The gemmules are represented in Diagram / by 

 the small circles marked with the small letters / to w. 

 The gemmules are seen to be traversing the space which 

 separates them from the germ-cells, and also stored up 

 in the latter. This double representation is explained 

 on p. 125. 



The space between the circle of somatic cells and the 

 central germ-cells in Diagram / has been introduced 

 for the sake of clearness. In higher animals the distance 

 which the gemmules would be compelled to travel in 

 order that the change in a brain-cell may be registered 

 in a germ-cell, would be relatively greater than that 

 represented in the diagram. 



With this hypothesis every somatic cell is a germ-cell, 

 while the germ-cells proper are merely the meeting-place 

 for the germs of somatic cells. Because every part of 

 the body is thus supposed to reproduce itself, Darwin 

 called his hypothesis Pangenesis. Each germ-cell is sup- 

 posed to be, as it were, an extract of the whole body ; 

 a microcosm, in which every cell that takes part in the 

 composition of the organism is represented. 



The first difficulty which this hypothesis encounters 

 is the almost infinite complexity of a germ-cell which 

 contains a material particle, a representative or gemmule, 

 from every somatic cell of one of the higher animals. 

 The countless number of cells in the human body may 

 be imagined from the fact that it would require over ten 

 million red blood corpuscles, lying flat, one deep, to 

 cover an area one inch square. And yet every single 

 blood corpuscle, although not exactly a cell itself, is the 

 product of a single cell. 



But this is not all ; for we must also suppose that each 

 cell of every stage of development, and of the cell- 

 generations which succeed each other during maturity, 

 is also represented in each germ-cell, and is the material 

 cause of the reappearance of such stages and such genera- 



