IOWA ACADEMY OF SCIENCES. 31 



plasm by conditions which affect the development of par- 

 ticular parts of the body; that is, exercise of an arm, 

 although modifying the structure of that arm cannot 

 affect especially the particular' part of the germ plasm 

 which represents the arm, that is which controls the devel- 

 opment of the arm. 



Weismaun's theory of the continuity of the germ plasm 

 cannot, in its essentials, be classed either with the pre- 

 formation or epigenesis theories; for this division is based 

 on the nature of the basis of heredity, not on its source. 



But Weismann has built upon this original idea a theory 

 of the structure of this germ plasm that presents an 

 extreme case of preformism, and which has to a great 

 extent driven other theories of this class from the field. 

 He identifies the germ plasm with the chromatin of the 

 nucleus, and sees in the complicated process by which this 

 chromatin is split up during cell division the mechanism 

 b}^ which the proper portions of the germ plasm (or chro- 

 matin) are accurately allotted to those parts of the body 

 whose development they control. 



k\\ preformation theories are open to the general philo- 

 sophical objection that they try to explain a complicated 

 bodily structure by an equally and perhaps even more 

 inexplicable complexity in the germ plasm, instead of 

 explaining this complexity by showing how it might have 

 arisen from a relatively simple condition in conformity 

 with known laws. If Weismann's views of the structure 

 of the germ plasm are true, we have presented to us the 

 problem of explaining how its inarvelous complexity could 

 have arisen; a problem that is not rendered less great cer- 

 tainly be removing the structures from the region of the 

 visible to that of the invisible. 



All preformation hypotheses attribute, as will be at 

 once seen, the major influence during development to 

 heredity. As the individual is already represented in all 

 its parts in the germ, the environment plays a very sub- 

 ordinate part, its infiuence being confined mainly to the 

 providing of nourishment for growth. Of course, all 

 external infiuence is not excluded, but it must be looked 



