288 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



equable temperature and in filling all the basins with cold water. Accordingly the last 

 regions in the oceans to come under the influence of glacial climates must be the shallow 

 waters of the equatorial area. The proof of this conclusion is seen in that the last stand 

 made by the marine Paleozoic world is recorded in the deposits of Tethys, the great Mediter- 

 ranean sea of PeriTiic time. It is also here that we find nearly all of the Paleozoic shallow- 

 water hold-overs in the succeeding period, the Triassic. 



The cooled but not frigid climate that followed the magnificent mountain-making at 

 the close of the Cretacic also produced striking changes in the organic world. These 

 changes were less marked than those of Permic time and more noticeable among the land 

 animals than those of the marine waters, affecting especially the over-specialized, large, 

 thick-shelled, and degenerate stocks. 



Great changes were again produced among the large land animals of the world, as 

 well as among those of the polar and temperate oceanic waters, by the glaciation of Pleisto- 

 cene time. The present shallow waters of the equatorial region still maintain the late 

 Tertiary faunas, and Africa is the asylum where the higher Pliocene land animals have 

 been preserved into our time. 



What the effects of the Proterozoic glacial climates were upon the Uving world of that 

 time it is impossible to say, because we have as yet discovered but little of the organic 

 record. The apparently sudden appearance of life at the base of the Cambric is partially 

 explained by the widespread absence of the marine Proterozoic record, an era during which 

 the nuclear portions of the continents appear to have been decidedly emergent for a very 

 long time. 



The marine "life thermometer" indicates vast stretches of time of mild to warm and 

 equable temperatures, with but slight zonal differences between the equator and the poles. 

 The great bulk of marine fossils are those of the shallow seas, and the evolutionary changes 

 recorded in these "medals of creation" are sUght throughout eternities of time that are 

 punctuated by short but decisive periods of cooled waters and great mortahty, followed by 

 quick evolution, and the rise of new stocks. The times of less warmth are the miotherm 

 and those of greater heat the pliotherni periods of Ramsay (1910: 15). 



On the land the story of the cUmatic changes is different, but in general the equability 

 of the temperature simulates that of the oceanic areas. In other words, the lands also 

 had long-enduring times of mild to warm climates. Into the problem of land climates, 

 however, enter other factors that are absent in the oceanic regions, and these have great 

 influence upon the climates of the continents. Most important of these is the periodic 

 warm-water inundation of the continents by the oceans, causing insular cUmates that are 

 milder and moister. With the vanishing of the floods somewhat cooler and certainly drier 

 cUmates are produced. The effects of these periodic floods must not be underestimated, 

 for the North American continent was variably submerged at least seventeen times, and 

 over an area of from 154,000 to 4,000,000 square miles (Schuchert, 1910: 601). 



When to these factors is added the effect upon the cUmate caused by the periodic 

 rising of mountain chains, it is at once apparent that the lands must have had constantly 

 varying chmates. In general the temperature fluctuations seem to have been slight, but 

 geographically the cUmates varied between mild to warm pluvial, and mild to cool arid. 

 The arid factor has been of the greatest import to the organic world of the lands. Further, 

 when to all of these causes is added the fact that during emergent periods the formerly 

 isolated lands were connected by land bridges, permitting intermigration of the land floras 

 and faunas, with the introduction of their parasites and parasitic diseases,* we learn that 

 while the chmatic envkonment is of fundamental importance it is not the only cause 

 for the more rapid evolution of terrestrial Ufe. Unfortunately, the record of land hfe, 



*This subject is fully discussed by R. T. Eocles, m.d., in the following papers: " Parasitism and Natural Selection," 

 " Importance of Disease in Plant and Animal Evolution," " The Scope of Disease," and " Disease and Genetics." 

 Medical Record for July 31, 1909; March 16, 1912; March 8, and August 2, 1913. 



