54 THE FIRST PRINCIPLES OF HEREDITY 



{b) Darwin's Pangenesis. 



The theory next in time and importance is the theory of 

 Pangenesis, put forward by Charles Darwin as a pro- 

 visional hypothesis in his Variation of Animals and Plants 

 under Domestication (1868). He assumed that each cell of 

 the body during all its stages of development throws off 

 minute particles, the " gemmules," which are character- 

 istic of each such cell during its respective stages, and are 

 able to multiply and reproduce the cell with its qualities at 

 the propitious moment. The gemmules circulate through 

 the body, and accumulate at particular places of it — that 

 is, at those which give rise to buds, etc., but especially in 

 the sex-elements of the organism — which thus contain 

 representative gemmules from each unit of the body 

 during all its stages. In this way it is explainable why the 

 germ-cells are able to reproduce the parent with all its 

 characteristics. During Ontogenesis (the development of 

 the individual) each embryonic cell, as it divides, is entered 

 by the gemmules which represent the cells of the next stage 

 of development, and which by a sort of fertilization, as it 

 were, induce this cell to assume the structure of the body- 

 cell they represent. In the same way it must be assumed 

 that in the regeneration of lost parts, etc., the right kind 

 of gemmules is attracted by the cells which divide to 

 repair the loss. Some of the gemmules during the develop- 

 ment of the sex-cells may remain latent for one or more 

 generations. Thus, the sexual character of a grandfather 

 (beard, etc.) may be transmitted through his daughter to a 

 grandson ; or, in cases of Atavism or Reversion, traits of 

 far-back forefathers may reappear, showing that the gem- 

 mules representing those characters were dormant, and 

 became active once more in the individual which shows the 

 trait anew. Lastly, Darwin assumed that cells modified 

 by changed conditions of life will throw off modified 

 gemmules, such gemmules, of course, reproducing parts 

 modified in the same direction. 



