THE MECHANISM OF HEREDITY 77 



tor or the atom from- two points of view, either as a 

 mathematical concept or a physical reality. As a mathe- 

 matical concept it is the unit of heredity, and a unit in 

 any notation must be stable. If one creates a hypothetical 

 unit by which to describe phenomena and this unit varies, 

 there is really no basis for description. He is forced 

 to hypothecate a second fixed unit to aid in describing 

 the first. 



The point at issue in this connection may be explained 

 as follows : Characters do vary from generation to gen- 

 eration, and the question to be decided is, how much of 

 this variation is due to the recombination of factors (con- 

 sidered now as physical entities) and how much is due to 

 change in the constitution of the factors themselves. The 

 obvious way to determine such a matter is first to appeal 

 to Nature and see whether it is possible for characters 

 to have a long' period of stability under any conditions ; 

 and, second, to investigate the stability of characters 

 when the environment is comparatively constant and 

 change due simply to recombinations of heterozygous 

 factors is eliminated. 



Of the results of the appeal to Nature only one need 

 be mentioned. Wheeler 215 has found that ants preserved 

 in amber of the Oligocene period, fossils which are better 

 preserved than any others, and which are thought to be at 

 least 3,000,000 years old, are practically identical with 

 living species. The only points of variance to be observed 

 are slight differences in shade of color, something prob- 

 ably due to the mode of preservation. Thus it is clear that 

 organic characters may remain stable for periods of time 

 so great as to be beyond our powers of realization. 



Investigations as to the effect of selection on homo- 



