INTRODUCTION xxvii 



the returning host, and hence migration became a fixed 

 and necessary habit. The range and increased breed- 

 ing area increased in waves mechanically. The exodus 

 would naturally be to the new-found land of plenty. 

 The birds of last year would return to their old breed- 

 ing station : their young also. But these would be 

 driven by the old birds further a-field and establish new 

 colonies ; for it is well known that birds will drive away 

 their young and jealously defend their appropriated 

 breeding territory against all-comers. 



Mr. Pycraft makes a laudable attempt to combat the 

 contention, which has been and still is urged, to the 

 effect that small variations in animal colour and form can 

 have no value as features for selection in the struggle 

 for existence. He urges that even if this be true in 

 many cases, yet such small variations will not be sup- 

 pressed by selection adverse to them. They are free 

 to take their course unless and until of such magnitude 

 as to be either checked by natural selection or favoured 

 by it. In illustration of his contention, he cites various 

 facts in regard to the evolution of the skull and of par- 

 ticular skull bones and other skeletal structures in birds. 

 This view appears to me to be well worthy of con- 

 sideration. 



In regard to the theory of sexual selection the original 

 views of Darwin and those of Wallace are cited as well 

 as the more serious criticisms of these views advanced 

 by other observers. In the light of some recent work, 

 Mr. Pycraft is led to hold that some slight re-setting of 

 Darwin's views in their application to birds is necessary. 

 Without endorsing his view of the absence of selective 

 action in certain cases, I may draw the reader's attention 

 to his argument. Since it is now known that birds of 

 the most sober hues affect displays of a character in- 



