572 EVOLUTION 



individual is formed. Thus a part of the fertilized egg from which 

 an individual develops becomes somatoplasm and the other part 

 remains germ-plasm. The body of the individual protects and 

 feeds the germ-plasm but has very little to do with determining 

 the constitution of the germ-plasm in respect to the factors 

 contained for characters. This means that germ-plasm remains 

 about the same throughout successive generations in respect to 

 the factors for characters contained. Thus the germ-plasm we 

 inherited from our parents contains only the factors for characters 

 that our parents received from our grandparents and our grand- 

 parents received from our great-grandparents and so on. This 

 means that the variations a plant or animal may have due to 

 environment neither adds nor takes away any factors from the 

 germ-plasm. Such variations affect only the somatoplasm. 

 They disappear when the body of the individual dies, for there is 

 nothing in the germ-plasm to represent them and perpetuate 

 them in succeeding generations. Such a view of course gives no 

 sanction whatever to the theory of acquired characters. Only 

 variations that have their origin in the germ-plasm are hereditary. 



So far as we have considered the theory, it may appear that 

 such a theory provides no way for hereditary variations to arise. 

 Each generation is simply a duplicate of previous generations in 

 heritage. But there are two ways in which Weismann thought 

 variations may arise in the germ-plasm and result in variations 

 in the characters of the individuals developing therefrom. 



His idea was that the characters of an individual are represented 

 and determined by particles of chromatin in the germ-plasm. 

 These chromatin particles he called determinants. In fertiliza- 

 tion there come together in the fertilized egg the determinants 

 contributed by each of the parents and each parent contributes 

 enough determinants to produce all the characters of the indi- 

 vidual. In other words, an individual has determinants in 

 duplicate for each character. Since the parents are never 

 exactly alike and often they are quite different in their characters, 

 there must be a difference in their determinants. Also in one 

 parent certain determinants are active, while in the other parent 

 certain other determinants are the active ones. In one parent 

 the determinant for red flower may be active, while in the other 

 parent the determinant for some other color may be active. It is 

 obvious that in the association of the two sets of determinants in 



