28 THE THEORY OF THE GENE 



perience, somewhat incredulous in respect to any and all 

 theories that postulate invisible units. A brief examina- 

 tion of a few of the earlier speculations may serve to 

 make the difference between the old and the new proce- 

 dure more apparent. 1 



Herbert Spencer's theory of physiological units, pro- 

 posed in 1863, assumes that each species of animal or 

 plant is composed of fundamental units that are all alike 

 for each species. The elements concerned are supposed 

 to be larger than protein molecules and more complex in 

 structure. One of the reasons that led Spencer to this 

 view is that any part of the organism may in certain 

 cases reproduce the whole again. The egg and the sperm 

 are such fragments of the whole. The diversity of struc- 

 ture in each individual is vaguely ascribed to a "polar- 

 ity" or some sort of crystal-like arrangement of the ele- 

 ments in different regions of the body. 



Spencer's theory is purely speculative. It rests on the 

 evidence that a part may produce a new whole like itself, 

 and infers from this that all parts of the organism con- 

 tain material out of which a new whole may develop, but, 

 while this is, in part, true, it does not follow that the 

 whole must be made up of a single kind of unit. Our 

 modern interpretation of the ability of a part to develop 

 into a new whole must also assume that each such part 

 contains the elements for the construction of a new whole, 

 but these elements may be different from each other, and 

 to this difference the differentiation of the body is re- 

 ferred. So long as a complete set of units is present, the 

 power to produce a new whole is potentially given. 



Darwin's theory of pangenesis, proposed in 1868, ap- 

 pealed to a host of different invisible particles. The 

 theory states that minute representative elements, called 



i A full discussion of earlier theories is given by Delage in Eeredite and 

 by Weismann in the Germ-Plasm. 



