Causes Affecting Distribution 27 



confined to circumscribed areas could exist as well in other parts of the world 

 where climatic and food conditions are practically similar, provided the avenues 

 for reaching them were open. For instance, the abundant fresh-water bodies 

 of the New World, teeming with fishes and other aquatic life, seem admirably 

 adapted to support a varied Kingfisher fauna, and doubtless our Troupials 

 (Icterida) could change places with the Old World Starlings (Sturnidce'), or our 

 Wood- warblers (Mniotiltida) with the Old World Warblers (SylviidcR). These 

 groups, and of course many others could be mentioned, appear to have reached 

 their present standing in approximately the same geographical areas they now 

 occupy, and long since all land connection between their respective habitats has 

 been cut off. 



In the light of geological evidence it is beyond dispute that in the past numer- 

 ous large land masses have been many times joined together and many times 

 rent asunder. In this way continents as well as lesser land areas have been suc- 

 cessively joined and cut off from one another, and each junction has left the way 

 open for an exchange of life forms. North and South America have been many 

 times severed and united; Australia by a relatively slight subsidence has lost 

 New Zealand, and Madagascar was undoubtedly at one time a part of the conti- 

 nent of Africa. Both New Zealand and Madagascar were stocked with certain 

 animal forms while they retained connection with their parent masses which the 

 sea barrier has since prevented from commingling, at least to any great extent. 

 The important bearing of these facts on distribution is obvious. 



Another and very important geological event was the glacial epoch. This 

 vast ice-mass, sweeping down for hundreds of miles from the pole, profoundly 

 modified the life, not only in the area actually covered by the ice, but far beyond 

 its actual border. Many forms must have been crushed out of existence, while 

 others, enjoying perhaps better means of migration, were pushed before it towards 

 the tropics, which resulted in intensifying the struggle for existence in an area 

 probably already well stocked. The forms of life from both sources that could not 

 readily adapt themselves to the changed conditions were pushed to the wall and 

 left little or no trace of their existence. By the recession of the ice, territory was 

 gradually reclaimed, which was occupied by the surviving forms, with the ex- 

 ception of those typically northern forms remaining permanently stranded in 

 southern mountain areas. 



To paleontology we are also indebted for some contributory data regarding 

 distribution, for while the fossil remains of birds are not very numerous, they are 

 often sufficiently so to show that many groups once enjoyed a much wider dis- 

 tribution than now. Paleontology thus makes plainer the possible lines of travel 

 by which the descendants of the ancient forms have reached their present loca- 

 tions. But this class of facts is far less important than those last considered, for 

 the paleontological record is less complete for birds than for almost any other 

 group. It is mainly of value in fixing the antiquity and affinities of certain groups, 

 and even here it is often distressingly meager. 



Of the causes controlling or influencing distribution it is generally admitted 

 that temperature and humidity are the chief factors, and, according to Merriam, 



