294 ANNALS NEW YORK ACADEMY OF SCIENCES 



terms of Northern derivation. Oceanic, desert or mountain barriers 

 have been much less efficient in limiting their range, and the efficiency 

 of the climatic factor is much more obvious than with mammals. Their 

 dispersal from a Holarctic center in successive waves of migration is 

 indicated by the dominantly Holarctic habitat of the highest and latest 

 developed groups, by the generally tropical habitat of archaic groups 

 often highly specialized, whose ancestors or 1'elatives are in many cases 

 known from the Holarctic Tertiaiy, and by the fact that the southern 

 continents are peopled, not by a series of dominant groups corresponding 

 to the Holarctic groups, evolved in a common Antarctic center, but chiefly 

 by groups of more or less tropical affinities and by a few northern groups 

 which have crossed the tropic barrier. There are many groups of birds 

 living to-day in the widely separated tropical regions whose ancestors 

 have not thus far been discovered in the Holarctic Tertiary. But they 

 correspond, both in distribution and in relative position in the classifica- 

 tion, with other groups which the geologic record proves to have origi- 

 nated by dispersal from Holarctica, and there is no valid reason for 

 assuming any other origin. The geologic record of Tertiary birds is far 

 more fragmentary than that of Tertiary mammals and especially in the 

 Nearctic region. 



It should further be observed that the perching birds represent the 

 primary adaptation from which the various specializations — terrestrial, 

 wading, marine, etc. — have diverged, and that, in consequence, these 

 divergently specialized forms retain various archaic features which have 

 been lost by the central group. 



The relations, dispersal and present distribution of birds are thus 

 wholly in accord with the principles here set forth. The detailed appli- 

 cation of these principles is beyond the limits of the present discussion. 



Dispersal of Amphibia 



The modern Amphibia include a few small and for the most part 

 highly specialized survivors of a group whose period of dominance dates 

 back to the Paleozoic. Of their Mesozoic and Tertiary ancestry almost 

 nothing is known. The Stegocephalia, the dominant Amphibia of the 

 Permian, were far less aberrant and much nearer to the contemporary 

 primitive Amphibia; their interrelationships are still far from being pre- 

 cisely definable, and, until these are better understood, it is futile to dis- 

 cuss the evidence which they may furnish as to former geographic con- 

 nections. 



The distribution of the modern Amphibia is often notably discontinu- 

 ous, and in the absence of evidence from extinct types as to the real 



