INTRODUCTION. vii 



The manner in which my journey was performed 

 has brought into some prominence the very interesting 

 but perplexing question of the value of apparently 

 specific differences. In consequence of my changing 

 the collecting-ground every two or three days, with 

 a few exceptions, throughout the season, by short 

 marches of from fifteen to thirty miles a day (moving 

 northwards in the summer of 1871 and southwards in 

 the spring of 1872), I was enabled to acquire a consi- 

 derable series of specimens of several species, ranging 

 perhaps, in some instances, over 100 or 150 miles of 

 country, exhibiting almost imperceptibly gradual but 

 extensive variation both in size and colour. In one or 

 two notable instances the range of this variation would 

 seem to include two or even three different forms, 

 which, if they had been found without their niter- 

 mediate connecting-links, would certainly have been 

 considered distinct species ; and even after a careful 

 comparison of an extended series, it must still remain, 

 in some cases, an open question whether they are or 

 are not entitled to specific rank. 



In the genera Amblyptilns, Hiibner, (Edemato- 



