INTERNAL CAUSES 531 



generation. Thus the germ-plasm remains about the same 

 throughout successive generations in respect to factors contained, 

 no new ones being added or old ones dropped. In other words 

 we inherited the same germ-plasm from our parents th'et they 

 inherited from our grandparents, and so on. Although the 

 body of each plant or animal is variously modified by the environ- 

 ment, there are no factors added to the germ-plasm to represent 

 them, and consequently they are not transmitted to the next 

 generation. They disappear with the body of the individual. 

 This theory is known as the continuity of the germ-plasm. The 

 theory also holds that only those variations whose origin is due 

 to variations among the factors in the germ-plasm are hereditary. 



Variations in the factors of the germ-plasm are accounted for 

 in two ways. First, the factors of the same nucleus are in com- 

 petition with each other for nourishment. The distribution of 

 food and water may so alter that factors previously well nourished 

 may lose in nourishment to the advantage of others which, previ- 

 ously poorly nourished, become much better nourished. Due to 

 such a change in nourishment factors previously inactive become 

 active and active ones inactive in the germ-plasm. It is obvious 

 that individuals inheriting germ-plasm in which such variations 

 have occurred will have corresponding variations in their char- 

 acters, and these variations are hereditary since their causes are 

 transmitted in the germ-plasm. 



Second, in fertilization, commonly there are brought together 

 germ-plasms of two parents that differ considerably in their 

 characters. The germ-plasms of such parents differ in factors for 

 characters and when brought together in fertilization, variations 

 may arise in the offspring due to the interaction of factors. Thus 

 the factor for red may dominate the factor for white, factor for tall- 

 ness may dominate factor for dwarf ness and so on, when such 

 contrasting factors are brought together in fertilization. The 

 interaction of factors may result in something different from 

 either parent, as red flowered offspring from white flowered 

 parents. 



External conditions, such as drought, shade, poor nourishment, 

 etc., which modify the body of the individual may so in- 

 directly affect the germ-plasm as to cause variations in its 

 factors and consequently variations in the characters of the next 

 generation of offspring, but the modifications of the body or, in 



