1912 ] Trotter, Relation of Genera to Faunal Areas. 165 



(4) — The greater antiquity of one genus as compared with 

 another would seem to be indicated by the larger number of its 

 species and their wider variation from one another, but this might 

 be offset by a greater resistance against disruption so that the genus 

 represented by only one species might be, in reality, quite as ancient 

 as the numerically high one. Furthermore we cannot know how 

 many forms may have died out in any genus. 



(5) - — A genus is definitely related to a geographical range, 

 while its several species are more definitely related to habitat 

 conditions within the range, especially the breeding area. 



(6) — A fauna is an expression of the temporary adjustment of 

 any group of living beings to given conditions of environment. 

 No single factor conditions its components or its boundaries. In 

 the sum of its conditioning factors character of vegetation is 

 probably the most important determining influence. Unquestion- 

 ably the changes which man has wrought upon the face of the 

 country by the clearing of forests and the development of agricul- 

 ture has profoundly influenced the distribution of many species 

 of birds. 



(7) — All species tend to spread, as their ancestral types have 

 spread, wherever suitable habitats are accessible to them. Search 

 for food, especially at the breeding season, is the motive. Heredity 

 has fixed the migratory impulse. The long dayhght of the north- 

 ern summer has probably had a determining influence in the 

 northward spread of ancient generic types and of their descendant 

 species. '^ 



1 E. A. Schafer, F. R. S. On the Incidence of Daylight as a Determining Factor 

 In Bird Migration. Nature, Dec. 19th, 1907. 



