THE CATEG< 



late as around a new center of equilibrium. Weismann 

 postulates a self-regulating power in the germ plasm 

 which keeps numerous minor fluctuating variations from 

 producing any essential modification of the stock. If, 



ity it may be permanent. The various mutants ol (kno- 



potency of giving rise to any of the others and which pre- 

 sent very different degrees of stability, may be due to 

 more or less stable forms which the germ plasm may as- 

 sume rather than the creation of new kinds of germinal 

 units. The stability of a variation may be due. however, 

 not so much to its extent as the analogy with the polyhe- 

 dron might lead us to expect as to its kind. Variations 

 which are physiologically congruent with the organized 

 structure of the germ plasm form stable races; those 

 which are not tend to become reduced sooner or later to 

 the norm through the regulatory activity of this sub 

 stance. . . 



The germ plasm may be conceived to exercise, m regard 

 to its variations, a kind of selective activity which may 

 manifest itself as a proneness of the organism to vary 

 along certain lines. It is well known that there are partic- 

 ular types of variation which crop out independently and 

 more or less frequently and may be faithfully perpetu- 

 ated. Polydactylism, split-hand and split-foot. ^'^^ 

 and melanism, the appearance of races of hairles> nmmu > 

 and glabrous plants, the development of U{ '[' Um ^ 

 peaches and peaches from nectarines, the origin o p< <>i u 

 flowers, etc., have occurred many times in independent 



It is of course possible that the mutations 0, . flft [* < *?* { j™" 

 result from the impurity of the stock. The species has e<> i • ' * 



