cH. XV] Segregation and Species 283 



the greatest advance that can be foreseen in this department 

 of physiology will be made when the nature of the inter- 

 action between the chemical and the geometrical phenomena 

 of heredity is ascertained. 



Bearing on the Theory of Evolution. 



The consequences of these discoveries to the general 

 theory of Evolution must be examined in reference to 

 concrete examples in order that the extent to which they.*^ 

 reach, and the limitations by which they are beset, may 

 rightly be apprehended. It is only by the detailed study 

 of actual cases of interrelation between kindred species that 

 the scope of Mendelism in this region of inquiry can be 

 measured and illustrated. I hope subsequently to publish 

 a separate volume^ in which this part of the subject will 

 be considered. Two features which emerge salient on such 

 a survey may be named at once. 



Fig. 38. The two " Cupid" types in F^ from Cupid x Bush (Sweet Peas). 

 I. The ordinary, prostrate Cupid. II. The erect Cupid. 



I. Mendelian Segregation and Species. 



First it is certain that segregation in countless instances 

 plays a part in the constitution and maintenance of charac- 

 teristics held by systematists to be diagnostic of species. 

 One has only to glance over trays of birds' skins, the 



* This volume which is designed to be in some respects a continuation 

 of the present book will be based on lectures given as Silliman Lecturer to 

 Yale University in 1907, under the title "The Problems of Genetics." 



