No. 510] HEREDITY AND VARIATION 



yet thoroughly analyzed facts that are difficult to inter- 

 pret on this basis, so that its general adequacy remains 

 to be determined. 



The work with Protozoa emphasizes further certain 

 important points regarding variations. The interest of 

 studies in variation lies mainly in the assumption that the 

 variations are heritable, supplying material for selection 

 and evolution; this assumption is openly or tacitly made 

 in most work on the subject. Yet when we actually de- 

 termine how far this is true, we find (as we have seen) 

 that in a pure race of infusoria all the differences be- 

 tween individuals are environmental and without signifi- 

 cance in inheritance. If we study these variations by 

 biometric methods and laboriously work out numerical 

 coefficients of variation (as the author has done on a large 

 scale in his original paper) we acquire data which are 

 perhaps not without some sort of interest, but which have 

 no bearing on inheritance or selection or evolution. The 

 ' ' standard deviation" and " coefficient of variation" ex- 

 press in a pure race mere temporary conditions, of no con- 

 sequence in heredity. If we could make all conditions 

 of growth and environment the same throughout our pure 

 race, all the evidence indicates that the standard deviation 

 and coefficient of variation would be zero, and this is the 

 positive value of their assistance in determining what 

 shall be the characteristics of the progeny. 



Even when we deal with mixtures of diverse races the 

 coefficient of variation is, as a rule, mainly determined by 

 differences due to growth and environment. 4 4 Wild" 

 cultures of Paramecium, consisting of many races, may 

 not give higher coefficients of variation than those ob- 

 tained from pure races. The coefficient of variation is 

 then by no means an index to the permanent heritable dif- 

 ferentiations in a collection ; its value may be high when 

 there are no such differentiations, or low when such dif- 

 ferentiations exist. The important question for all such 

 work is: What diversities are heritable, what are not? 

 This is an experimental question, and the role of statis- 



