228 THE FROG CHAP. xin. 



to discuss either the causes of variability or those which 

 determine the elevation of a variety to the rank of 

 a species : both questions are far too complex to be 

 adequately treated except at considerable length, and 

 anything of the nature of a brief abstract would only be 

 misleading.* 



* At a later stage the student is recommended to consult 

 Doncaster's Heredity in the Light of Recent Research (Cambridge 

 Manuals of Science and Literature, 1910), which deals with the 

 modern developments of these and other questions, such as 

 Mendelian Heredity. 



