508 THE AGE OF MAMMALS 



conditions. The plains and forest regions most densely populated with 

 quadruped life, such as those of the African plateaux of the present 

 day, are regions of moderate rainfall and even of prolonged summer 

 droughts. The regions less densely populated with quadrupeds are 

 those of heavy rainfall, of dense forests and vegetation, such as those 

 of the equatorial belt of South America or the Mango region of Africa. 

 Moisture and temperature are, therefore, to be first considered in relation 

 to forestation. 



Increased rainfall has many other effects: (1) it may diminish the sup- 

 ply of harder grasses to which certain quadrupeds have become thoroughly 

 adapted ; (2) it may at the same time produce new poisonous or deleterious 

 plants; (3) it may be the means of introducing new insects or other pests, 

 and new insect barriers ; (4) it may be the means of introducing new proto- 

 zoan diseases and new insect carriers of disease; (5) it may be the means of 

 erecting new forest barriers to migration, or new forest migration tracts 

 for certain Carnivora, such as the bears. 



It follows that periods of secular increasing moisture, such as the early 

 and mid-Pleistocene of the northern hemisphere is supposed to have been, 

 may have been unfavorable to certain large quadrupeds which had become 

 adapted to Pliocene conditions of semi-aridity, even prior to the advent of 

 extreme cold. As regards migration, Merriam observes that humidity is a 

 less potent factor than temperature in limiting the distribution of the Mam- 

 malia of North America. Thus many genera adapted to certain restrictions 

 of temperature ranged east and west completely across the American con- 

 tinent, inhabiting alike humid and arid subdivisions, but no genus adapted 

 to certain conditions of humidity is able to range north and south across the 

 temperature zones. 



Food supply and moisture. — Sheep and cattle owners of the North- 

 west have observed that the majority of poisonous plants are those 

 which flourish during moist seasons, such as the death camas (Zygadenus), 

 the larkspurs (Delphinium), the water hemlocks (Cicuta), the white loco 

 (Aragallus). 



It is a matter of universal observation that in tick- or insect-infested 

 countries dry seasons result in the reduction, moist seasons in the increase 

 of diseases: dry localities are favorable; moist localities are unfavorable. 

 Thus the tse-tse fly is not found in the open veldt; it must have cover. 

 Warm, moist, steamy hollows containing water, inclosed with forest growth, 

 are the haunts chosen. 



Ticks, even when non-infection-bearing, form absolutely effective bar- 

 riers to the introduction of quadrupeds into certain regions. In certain 

 forested portions of South and Central America they endanger human life. 

 In certain regions of Africa ticks are practically fatal to horses. As observed 

 bj^ Elliot, thousands of ticks would sometimes gather on a horse as the 

 result of a single night's grazing. The mane especially serves to collect 



