PREFACE. IX 



jections by Mr. Wallace and Professor Gill to the views there 

 formulated appear in " Natiu'e " of March 22, and June 7, 

 1883, respectively, and my rejoinders to the criticisms of 

 these gentlemen in " Nature " of April 26, and the " Proceed- 

 ings " of the Philadelphia Academy for November, 1883. To 

 these papers I must refer the reader for a purely technical 

 statement of the case. The classification of the " Transition " 

 tracts is largely that which has been proposed by Forsyth 

 Major, in " Kosmos " for 1884. 



Academy of Natural Sciences, Philadelphia, October, 1886. 



