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 following 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 frequently 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 tothe 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 heen ascertained to remain the same wherever 
it may grow. To determine this constant the temperature of 48° 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.’ 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 
1C, Hart Merriam, ‘‘Laws of temperature control ‘of the geographic distribution 
of terrestrial animals and plants,’’ Nat] 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. 
