114 



ANALYSIS OF THE ENVIRONMENT 



in many other data that suggest that ani- 

 mals with a higher rate of metabolism tend 

 to live for a shorter time than do others of 

 the same species with lower metabolic rates. 

 Frequently these data are not carefully 

 quantitative. It is, for example, rather gen- 

 eral experience that the rate of metabo- 

 lism of males is higher than that of females 

 and that the latter live longer. Fairly exact 

 information has been collected on this sub 

 ject for the cladoceran, Daphnia magna 

 The rate of heart beat gives a rough indi- 

 cation of the rate of metabolism of this or- 

 ganism (Table 8). 



15° than at 26° or 30° C. and much 

 greater in light than in darkness (Northrup 

 1926; MacArthur and Baillie, 1926). 



THE LIFE ZONE CONCEPT 



Merriam's so-called temperature laws on 

 which his life-zone system for North Amer- 

 ica is based, were stated in 1894. They 

 grew out of the idea of temperature sum- 

 mation and find their modem basis, in part, 

 in the hyperbolic temperature equations. 

 The forerunners of the scheme included the 

 system of faunal areas worked out by 

 American naturalists and Merriam's own 



Table 8. The Mean Age in Days and the Average Number of Heart Beats per Second, Times 



the Average Age for Males and Females of D. magna 



(Data from MacArthur and Baillie, 1926, 1929) 



* Data for 1929 taken at 18° C. 



In these tests the females lived approxi- 

 mately six days longer than the males, yet 

 the product obtained by multiplying the 

 average Hfe-time rate of heart beat by the 

 duration of life in days is of the same order 

 of magnitude for the two sexes in each 

 test. The greatest discrepancy given here is 

 that for 1929; the males lived 13 per cent 

 shorter time than did the accompanying 

 females and showed a life-time heart beat 

 of 2 per cent more; unfortunately, we can- 

 not give a statistical evaluation of these 

 results. Regardless of differences produced 

 by temperature or sex, the average total 

 number of heart beats for this race of 

 Daphnia was estimated to be on the order 

 of 15.4 X 10\ 



Not all the tests of Rubner's hypothesis 

 have given positive results. In one set of 

 experiments, aseptic cultures of Drosophila 

 were reared under controlled conditions, 

 and their total carbon dioxide was measured 

 as an indication of total metabolism. The 

 amount produced was not constant, as re- 

 quired by the hypothesis; it was greater at 



sound field studies on vertical zonation of 

 animals and plants in the Rocky Moun- 

 tains (Merriam, 1890, 1892, 1894, 1899, 

 1899a; Allen, 1892; Daubenmire, 1938). 



When isotherms were drawn through 

 localities said to have equal sums of effec- 

 tive temperatures, certain of these coincided 

 suggestively with the northern boundaries 

 of distributions of certain animals and 

 plants. This is summarized in Merriam's 

 first temperature "law," which states: "Ani- 

 mals and plants are restricted in northward 

 distribution by the total quantity of heat 

 during the season of growth and reproduc- 

 tion." A similar study of the mean tempera- 

 ture for the six hottest weeks of summer led 

 to the second "law," which is: "Animals 

 and plants are restricted in southward dis- 

 tribution by the mean temperature of a 

 brief period covering the hottest part of 

 the year." 



The division lines ran roughly east and 

 west over the plains and prairies, but made 

 a sharp dip southward in the mountains. 

 Along the western mountain and coastal 



