i 2 8 THE VARIATION OF ANIMALS IN NATURE 



we hold that the occurrence of the various grades of poly- 

 morphism is far more widespread and far more significant, 

 and whether we are considering groups such as colonies of 

 land snails which are distinguished by the varying proportions 

 of a number of characters or the statistical differences in the 

 occurrence of single characters, we cannot fail to be impressed 

 by the evidence for a process of multiplication of certain types 

 rather than their production en bloc. Nevertheless, if the 

 evidence from the facts of distribution suggests such a process, 

 it does not justify any conclusions as to how it took place. 



