THEORIES OF MIGRATION 27 



change in the gonads, and thus directly activated by 

 the sex hormones. It appears true that the migration 

 north from the winter home is instigated by this 

 factor, which is of powerful influence in maintaining 

 and directing present migration movement; but, 

 though a prime influence at present, this cannot be 

 considered as the ultimate underlying principle that 

 has given rise to migration as we now see it. 



The evolutionary line of the bird is tremendously 

 long: on the basis of present knowledge it is known 

 to go back through an enormous reach of time to 

 Archaeopteryx and Archaeornis of the Jurassic 

 period. 



That these earliest known birdlike creatures pro- 

 gressed through the air is certain from their struc- 

 ture, but the extent of their powers of flight is prob- 

 lematical. In the Cretaceous, however, we find in 

 Ichthyornis a bird with ability to fly strongly de- 

 veloped, and the flying type has been the dominant 

 one in the millions of years during which our present 

 avian forms have been evolved. The evidence re- 

 corded by fossils indicates that many of our modern 

 types of birds were found during the Miocene. It 

 seems probable that the forms shown in many, if not 

 most, of our modern genera, were in existence at 

 that time, though our fossil record is highly incom- 

 plete. Bones of many birds not certainly distin- 

 guishable from those of living forms are found in the 

 early part of the Pleistocene. 



