THE GERM-PLASM THEORY 



367 



subject in the course of their development to different modifying 

 influences. But this is not the case, at least not to the extent tliat 

 this supposition would necessitate. Can it be supposed that, for 

 instance, the jumping legs of the water-flea {Gammarus) are a neces- 

 sary consequence of the somewhat divergent form of the segments 

 from which they grow 1 A direct proof to the contrary may be found 

 in 'Heterotopia,' for in the place where a posterior limb, modified 

 for holding the eggs, normally occurs in the crab an ordinary walking- 

 leg may exceptionally develop (Fig. 90, Bethe), or an appendage 

 resembling an antenna may take the place of an extirpated eye 

 (Herbst). But if there were really only one determinant in the 

 germ-plasm for all the appendages these would of necessity be 



*»j>v->r. 



Fig. 50. The Common Shore-Crab {Carcinus nuc-nas), seen from below, with 

 the abdomen forced back. In place of the swimmeret, which ought to be borne 

 by the fifth abdominal sv/immeret, a walking leg has grown on the left side, 

 and one which jiroperly should belong to the right side (6). 1-5, thoracic 

 limbs, jjs 1-4, swimmerets of the right side. s6, 57, posterior segments of 

 the abdomen. After Bethe. 



all alike, apart from the larger or smaller differences which might 

 be stamped upon them by growing from segments different in 

 size and in nutrition. Such differences, however, are far from being 

 sufficient to explain the great deviations seen among the appendages 

 of most kinds of Crustaceans, and still less to explain their adaptation 

 to quite different functions. 



It need not be imagined that my argument can be controverted 

 by saying that one appendage-determinant in the germ may split 

 itself in the course of development into a series of different appendage- 

 determinants. The question would then arise, How is it able to do so? 

 And the answer can be no other than that the single first determinant 

 had within it several different kinds of elements, which subsequently 



