58 HEREDITARY CHARACTERS 



For a theory which allows of the transmission of acquired 

 characters from parent to offspring we must turn again to 

 Darwin's theory of pangenesis. We have seen that this 

 theory assumes the presence in the germ-cell of innumer- 

 able gemmules, each of which represents or determines the 

 development of a cell similar to a pre-existing cell in the 

 bodies of the parents. This theory assumes further that 

 gemmules are produced in the cells forming the various 

 tissues and organs of the body, and migrate thence into the 

 germ-cells that are contained in the organism. Thus any 

 changes brought about by environment, or in fact by any 

 external influence, might, according to this theory, be 

 represented in the germ-cells, and acquired characters might 

 thus be transmitted to the offspring. 



There are of course many modifications of this theory. 

 For instance, de Vries assumes that what . he called 

 " pangens " (which seem to correspond more or less to 

 Darwin's gemmules) are contained in the nucleus. He 

 does not believe, however, that the pangens are cell-germs 

 as Darwin assumed for his gemmules, but that they are 

 the ultimate units containing potentialities of particular, 

 hereditary characters, and that from these units the cells 

 forming the body are built up during the process of 

 development. This theory of de Vries, which has been 

 accepted by many biologists, partakes both of the idioplasm 

 theory of Nageli, and of Darwin's theory of pangenesis. It 

 assumes of course that the idioplasm is made up of 

 pangens which migrate from the nucleus, and in growing 

 and multiplying build up the cytoplasm. 



We are faced then with two opposing theories ; one the 

 theory of Weismann that no acquired characters can be 

 transmitted from parent to offspring ; the other, the theory 

 of Lamarck that acquired characters may be and are 

 transmitted, and that the transmission of these acquired 

 characters has played an important part in the process of 

 evolution. The value of these opposing theories will be 

 discussed later. 



