400 BIOLOGY ANY ITS MAKERS 



animals, even the highest developed, begin in a fertilized egg, 

 that structure, minute as it is, must contain all hereditary 

 qualities, since it is the only material substance that passes 

 from one generation to another. This hereditary substance 

 is the germ-plasm. It is the living, vital substance of organ- 

 isms that takes part in the development of new generations. 



Naturalists are agreed on this point, that the more com- 

 plex animals and plants have been derived from the simpler 

 ones; and, this being accepted, the attention should be fixed 

 on the nature of the connection between generations during 

 their long line of descent. ' In the reproduction of single- 

 celled organisms, the substance of the entire body is divided 

 during the transmission of life, and the problem both of 

 heredity and origin is relatively simple. It is clear that in 

 these sinsjle-celled creatures there is unbroken continuitv of 

 body-substance from generation to generation. But in the 

 higher animals only a minute portion of the organism is 

 passed along. 



Weismann points out that the many-celled body was 

 gradually produced by evolution; and that in the trans- 

 mission of life by the higher animals the continuity is not 

 between body-cells and their like, but only between ger- 

 minal elements around which in due course new body-cells 

 are developed. Thus he regards the body-cells as constitut- 

 ing a sort of vehicle within which the germ-cells are carried. 

 The germinal elements represent the primordial substance 

 around which the body has been developed, and since in all 

 the long process of evolution the germinal elements have been 

 the only form of connection between different generations, 

 they have an unbroken continuity. 



This conception of the continuity of the germ-plasm is 

 the foundation of Weismann's doctrine. As indicated before, 

 the general way in which he accounts for heredity is that the 

 offspring is like the parent because it is composed of some of 



