92 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



A general migration to some distant region where conditions are more favorable would probably 

 have left a well-defined trail in the traditions of the whole region. Numerous traditions of local 

 migrations are known, but all should be scanned with care before acceptance. It seems to the 

 authors that a much more reasonable explanation of the known phenomena is this: If the rainfall 

 slowly decreased, conditions must have become very gradually more severe. More and more 

 frequent droughts and accompanying starvation periods would result, during which the weaker 

 members of the tribe would perish, not altogether from starvation, but from the reduction of 

 their powers of resistance to disease, cold, and other hardships through want of sufficient nourish- 

 ment. Thus the general physique of the tribe would be preserved by the weeding out of the 

 unfit instead of weakening the physique of the tribe as a whole. As the severity of such droughts 

 increased it is probable that minor wars for the possession of the small, better-watered tracts would 

 occur, still further reducing the various tribes and decreasing the aggregate population of the 

 region. Occasional minor epidemics would be apt to reduce still further their numbers, especially 

 if they occurred during periods of drought. Thus it is reasonable to suppose that as a natural 

 result of desiccation the population decreased so gradually that the decline could be discovered 

 only by very accurate statistical records or by a general comparison of the numbers hving in the 

 region at widely separated periods. In this way the depopulation would progress slowly by 

 natural processes and therefore would not attract the attention of the inhabitants and would 

 leave little impression in their legends or traditions. The remnant of the population would 

 gradually move in small bands to situations favorable to agricultural pursuits, thus becoming 

 widely dispersed. The foregoing changes would be expected to occur in a region which was 

 slowly drying up, and present conditions are just such as one would be led to expect. Hence it 

 seems very prol)able from the archeologic evidence that there has been progressive desiccation." 



It is interesting to find that in the publication under discussion botanical evidence receives 

 considerable attention, one of the joint authors. Professor Robbins, being a botanist. On page 56 

 he discusses a matter of considerable importance. My own notes contain many references to 

 phenomena identical with those he describes, although I have not discussed them so far as New 

 Mexico is concerned for the same reason which makes Professor Robbins hesitate, that is, because 

 the number of exact observations is limited; yet the fact that his observations and my own 

 agree so closely adds to their value.* 



"While it is true that during 3,000 years some species may be altered to a slight extent, others 

 may be introduced by various means, and others may come into existence suddenly (mutation), 

 and that the relations of formations and associations of plants may have changed in some measure, 

 yet it is highly improbable that there has been a marked and widespread modification of the 

 flora within that time. However, the relation of the two principal plant formations of the region 

 seems to afford some evidence of progressive climatic change. This may be seen in the stress 

 zone between the pinon pine-cedar formation and the rock-pine formation. Pinon pines and 

 cedars grow in drier situations than do rock pines. In the area under discussion rock pine occurs 

 on the higher parts of the mesas, back toward the mountains, while pinon pine and cedar are 

 confined to the lower portions, down toward the rim of the Rio Grande Canyon. At a distance 

 of 1 to 3 miles back from the Rio Grande the two formations meet and here there is a battle for 

 occupancj" of space. If in this struggle between these two plant formations the pinon pine-cedar 

 formation is the successful competitor and gradually encroaches on the rock-pine formation, and 

 if such encroachment is widespread, this condition probably indicates progressive desiccation of 

 the country. That is the condition in this region. If the rock-pine formation were extending into 

 the territory of the formation below it, there would be rock-pine seedlings as outposts of the 

 invasion, and their presence would be evidence that conditions in the new territory were favorable 

 for their growth. From the lower extension of the formation rock-pine seedlings are almost 

 entirely absent. The outermost individuals are large trees, in many cases the largest of the 

 formation, possibly several centuries old, indicating that in the early stages of their growth 

 conditions were more favorable for the species to obtain a .start and that no such favorable period 

 has occurred since. Pinon-pine and ceclar seedlings do occur at the stress zone, although not in 

 greater abundance than at any other point in the formation. The whole aspect of the line of 

 stress between these two formations shows that the pinon pine-cedar formation is encroaching 

 on the rock-pine formation, a condition which would not exist unless there is progressive desicca- 

 tion which is tending to make the debatable territory unfavorable for the rock pines and better 

 suited for pinon pines and cedars." 



* Robbins: Physiography of the Rio Grande Valley, p. 56. 



