LIFE ZONES OF HUMBOLDT AND OF MERRIAM. 29 



Recognizing heat as the prime factor in the distribution of plants, 

 this great investigator of nature established the boundaries of plant 

 zones by connecting the points having the same mean annual tempera- 

 ture. The resulting isothermal lines denote broadly the limits within 

 which certain plants find their demands upon this factor satisfied. He 

 established for plant as well as for animal life the folio wing life zones: 

 The Boreal, or Northern; the Austral, or Southern, and the Tropical 

 Zone. The borders of these zones rarely conform with the parallels 

 of latitude, but f requentl y diverge widely from them, according to the 

 elevation of the land, moving northward when they pass over lowlands, 

 moving southward when running over the mountains, and also affected 

 by the prevailing aerial currents and proximity to the sea. Since, how- 

 ever, two points of the same mean annual temperature may present 

 wide differences in the extremes of annual, monthly, or daily tempera- 

 ture, and since the physiological functions and the development process 

 must be correlated with these conditions, the zonal divisions established 

 upon these isothermal lines are in a high degree faulty. 



LIFE ZONES ESTABLISHED BY MERRIAM. 



The amount of heat required to accomplish the cycle from germina- 

 tion or the first movement in the unfolding bud to the maturity of the 

 seed has been called the physiological constant of the species, which 

 for a given species has been ascertained to remain the same wherever 

 it may grow. To determine this constant the temperature of 43 F. 

 (6 C.) has been assumed by physiologists in general as the lowest 

 point at which germination and bud movement takes place. As to the 

 method of establishing this physiological constant opinions differ 

 widely, some investigators believing that the desired coefficient is 

 obtained by multiplying the mean temperature of a certain period by 

 the number of days, while others find it in the sum of the maxima 

 shown by the thermometer exposed to the sun (maxima of insolation). 



Merriam recognizes it as a law that the physiological constant rests 

 upon the sum of the mean daily temperatures during the cycle of 

 vegetation. 1 By adding the mean daily temperatures above the 

 assumed zero point of vegetation at numerous stations of observa- 

 tion from spring until such time in the fall as the temperature again 

 falls to the zero point, and connecting stations of the same sum 

 of these effective temperatures, lines are established which are 

 regarded by Merriam as determining the northern limit of 

 the species. In its southward distribution he finds a barrier in the 



1 C. Hart Merriam, "Laws of temperature control of the geographic distribution 

 of terrestrial animals and plants," Nat'l Geogr. Mag., vol. 6, pp. 229 to 238, 3 col. 

 maps, 1894; The geographic distribution of animals and plants in North America, 

 Yearbook Dept. Agr. for 1897, pp. 203 to 214; Life zones and crop zones of the United 

 States, Bull. No. 10, Div. Biol. Surv., U. 8. Dept. Agr. 



