OF LIFE IN GENERAL. 



325 



From his extensive researches on the variation and 

 distribution of mammals and birds in North America, 

 J. A. Allen * has been able to arrive at several general- 

 ised conclusions concerning the geographical distribu- 

 tion of local races. Thus he finds that, as a rule, the 

 mammals and birds of North America increase in size 

 as we pass from the southern towards the northern 

 regions. In the accompanying table are given the 

 mean values obtained by him for the length of body, 



stretch of wings, and length of tail of seven different 

 species of birds. In the middle portion of the table are 

 given the actual values (in inches) for the Southern 

 races, from Florida, whilst in the right half are given 



* Bull. Mus. Comp. Zool. Harvard, vol. ii. p. 161-490; also Cope's 

 "Factors of Organic Evolution," p. 45. 



