320 PAST CLIMATES AND CLIMAXES. 



life under which each culture had its rise and decay. In so far as our chief 

 problem is concerned, it is evident that the appearance of man introduced a 

 new factor in the development of vegetation. As such a factor, man has 

 certainly not yet wrought his maximiun effect, though he has probably 

 developed all or nearly all of the kinds of effects. From the standpoint of 

 succession as well as that of evolution, man marks the beginning of a char- 

 acteristic period. It is unnecessary to point out the unique value of the Human 

 period as the link between the present and the past. For oiu* purpose, its 

 greatest value lies in the fact that the processes of our present are those of 

 its immediate past, just as these must have repeated and reproduced those of 

 still earlier times. (Osborn, 1915.) 



Culture relicts occur in both strates and stases, but the latter are usually 

 much more significant, because of their evidence of sequence. Culture stases 

 may be formed by calcareous deposits, as in the Cave of Castillo mentioned 

 earlier, or by deposit in water, such as occurs in the case of lake dwellings 

 and in peat deposits. Surface stases arise from the abandonment of hmnan 

 dwellings, monuments, etc., similar to those described by Huntington for the 

 recent Mayan and Pueblan cultures. Such stases frequently become buried, 

 and new stases arise upon them in sequence, as at Cnossus, Hissarlik, and 

 Mycenae, where a series of stases epitomizes the Neolithic, ^gean, and early 

 Grecian periods of culture. The environic relations of these cultures are still 

 to be analyzed. Such an analysis has been made by Huntington (1914 : 47, 

 175) in the case of the ancient peoples of Mexico, Arizona, and New Mexico. 

 In the arid regions the size, location, and nature of ruined villages indicate 

 the development of three ancient cultiu-es, the Hohokam, the Pajaritan, and 

 the Pueblan, all dependent upon agriculture. Each of these appears to have 

 waxed and waned, or disappeared, in consequence of moist and dry periods. 

 The desert evidence of such cycles is supported by the curves of growth and 

 rainfall as shown by Sequoia in California, and is checked by evidence of similar 

 cultural pulsations in the Mayan civilization of Yucatan, Guatemala, and 

 Honduras. In the desert, however, moist periods were times of expansion, 

 and dry ones of restriction, while in the region of tropical forest the reverse 

 seems to have been true (plate 59, a, b). 



CAUSES OF CLIMATIC CHANGES. 



Kinds of causes. — ^The principle of uniformity assumes that the causes of 

 climatic change in the past were the same as the causes which produce changes 

 at present. This naturally narrows the question to those forces which can 

 be observed to have a causal relation to climate at present. Huntington 

 (1914 : 234) has recognized this fact in distinguishing between " a highly theo- 

 retical conception, such as the precession of the equinoxes or the abstraction 

 of carbon dioxide from the atmosphere," and "an observational conception, 

 such as the climatic effect of the altitude and form of the lands, or the effect 

 of changes in solar radiation upon terrestrial temperatm-e." The latter alone 

 seem fuUy able to explain past climates and vegetation upon the assumption 

 of uniformity of causes. Hence, no further attention will be paid here to 

 Croll's theory of the precession of equinoxes and the Arrhenius-Chamberlin 

 carbon-dioxid theory. These and other theoretical conceptions are discussed 

 by Chamberlin and Salisbury (2:93, 660; 3:426, 432), Hann (1908:373), 



