374 



MODERN THEORIES OF HEREDITY. 



characters are inherited, differences may be expected between 

 the two sets of plants, those raised from " dry " seed having a 

 tendency to exhibit the " dry " type of leaf, and vice versa. As 

 a fact, no such differences can be detected, and we must admit 

 that if acquired characters are inherited at all, it is to a much 

 smaller extent than is commonly supposed — so small, in fact, 

 that direct experiment has never clearly revealed its occurrence. 

 Facts such as these have led to a very general acceptance of 

 Weismann's theory, the essential features of which are indicate.' 

 in Fig I. illustrating the "'continuity of the germ-plasm." 



While we may admit the 

 main idea involved in Weis- 

 mann's hypothesis, it is difficult 

 to avoid the speculation that ex- 

 ternal agencies may to some 

 small extent influence the germ- 

 cells as well as the body cells. 

 A rigid conformity with the 

 expectations of Weismann's 

 theory is obviously not attained 

 in the case of such plants as 

 hegonias, where the detached 

 leaves are capable of giving rise 

 to normal plants, although con- 

 taiiii)ig no gcnii-cclls nor, nor- 

 inallv, a)icesfors of gcrin-cells. 

 In various plants and animals 

 we find that almost any part of 

 the organism in the same way 

 may give rise to a whole new 

 individual, including the germ- 

 cells. Such facts are certainly 

 in strong opposition to Weis- 

 mann's theory, continuity of the 

 germ-plasm not occurring in the 

 sense in, which that phrase is 

 usually iniderstood, and no 

 sharp distinction being found 

 between reproductive and 

 somatic tissues. 



W^ith these ])reliminary re- 

 marks we can pass on to a con- 

 sideration of the two modern 

 views of heredity which form 

 the main theme of my paper. 

 As these two views, in spite of 

 efforts to reconcile them, are in 

 direct conflict with one another, 

 it will be necessary to consider 

 them separately. 



line of suo- 

 oesslon of 

 germ cells. 



line of suo- 

 oesBlon of 

 somatic oellB. 



Fig. I. 



