Earlier Theories of Heredity ii 



tion. Such an opinion may be true with regard to 

 specific characters, because deviations from the specific 

 type occur in such rare cases that they cannot hold their 

 ground against the large number of normal individuals, 

 but the case is different, as he claims, with those minute 

 differences which are characteristic of individuals, 

 because every individual possesses them, although 

 of different kind and degree. He states that cross- 

 breeding between all the individuals of a species is 

 impossible, hence the obliterating of individual differ- 

 ences is impossible. 



According to Weismann, therefore, individual varia- 

 tions, and therefore the possibility of natural selection, 

 originate in the sex process. Sex reproduction originates 

 no absolutely new characters, but merely makes possible 

 innumerable combinations of characters. It is evident 

 that the pairing individuals must differ to some extent, 

 if the result is to be a new combination of characters. 



This raises the question of the origin of differences 

 in the first sexual individuals. Sexual reproduction 

 was derived from asexual reproduction, and, according 

 to the theory, must have begun with variable material. 

 How did these individual differences arise ? Weismann 

 offers the following explanation. He had stated that 

 environment has no modifying influence upon the germ 

 plasm in general, but says that "in the lowest one- 

 celled organisms the case is entirely different." In 

 this case, as he puts it, "parent and offspring are in a 

 certain sense one and the same thing," and germ plasm 

 and body plasm have not been differentiated. In this 

 case, therefore, variations induced by external conditions 

 are hereditary because the germ plasm is the body plasm. 



