(b) — DeVries' Theory of Intracellular Pangenesis 

 or The Pangen Theory (1889) 



This theory may be summarized as follows: 

 Organisms are built up of unit characters which are 

 independently variable and heritable. They are repre- 

 sented in the germ plasm of the nucleus by definite bodies 

 (pangens) which constitute the chromosomes. These 

 pangens multiply in the idioplasm of the nucleus. Some of 

 them migrate into the surrounding cytoplasm, where they 

 become active and give it a particular character. A small 

 number of pangens always remain in the nucleus, and is 

 handed on from cell to cell by nuclear division. Into each 

 cell, as it is formed, a fresh migration of pangens occurs. 

 "It will be observed that DeVries drops Darwin's idea 

 of migration of the gemmules from the organism into the 

 germ plasm, and starts with these gemmules as permanent 

 constituents of the germ plasm." 



(c) — Weismann^ and Weismannism 



Weismann's main contribution to evolution literature 

 is his Germ Plasm Theory. This is primarily a theory of 

 heredity, and only when it is considered in connection with 

 other related matters does it become the theory of evolu- 

 tion called Weismannism (Fig. 9). 



Weismann (1834-1914) argued that as all higher ani- 

 mals and plants have originated from a single cell, a fertil- 

 ized egg, this minute structure "must contain all the heredi- 

 tary qualities, since it is the only material substance that 

 passes from one generation to another. This hereditary 

 substance is the germ plasm, and Weismann's special the- 

 ory is called the Continuity of the Germ Plasm.^ The fertil- 

 ized egg-cell is the result of fusion of two germ-cells, the 

 larger, the female egg-cell or ovum, and the smaller, the male 

 sperm-cell. Since the offspring often contains characteristics 

 of both parents it is also evident that each germ-cell must be 

 the bearer of special hereditary qualities. The body or 

 sotna, according to this idea, is merely "an offshoot — a house 

 built up out of a part of the substance of the original germ- 

 cell to shelter it until it decays, and the germ-cell is trans- 

 mitted to another house." 



(1) — Weismann studied medicine at Gottingen, and for over fifty years 



was teacher and professor of Zoology in Freiburg University. 



He was afflicted with an eye trouble which interfered with his 



work for man"y years. 

 (2) — This theory was first formulated by Francis Galton in 1875 in 



his "Stirp" theory of heredity. 



53 



