272 THE MERYCOIDODONTIDiE 



However, if the herds were already diminished or weakened from the physiological effects of such 

 vegetation, local extinction would be facilitated. Poisonous plants may have been numerous around 

 the water holes, for moisture favors their growth. Possibly certain of them were not injurious to the 

 adult while highly so to the young, even to the point of fatality. Another fact to be considered is 

 that certain of these deleterious forms of vegetation, for example Delphinium (larkspur) and 

 Aragallus (white loco), remain green, and hence seemingly edible, after the grasses have dried. 

 After snowstorms, when grasses are buried, the taller shrubs, often poisonous, will be greedily eaten. 

 A ruminant is not content until he has eaten his fill, so that he eats whatever is at hand until 

 satisfied. It is very probable, however, that the merycoidodonts in general were not ruminants and 

 hence not subject to all of the life conditions influencing the true ruminants. There are also certain 

 mechanically injurious plants, like porcupine grass, whose barbs enter the mouth, nose, throat, eyes, 

 ears, and sinuses and produce ulcers, generally causing death to the individual. 



Possibly the Miocene and Pliocene winters were sufficiently severe to bury local grassy areas, 

 and, if so, various herds reacted differently, as they do today. For example, horses have been known 

 to dig through three or four feet of snow for food, whereas cattle under the same circumstances have 

 starved to death. Again, periods of cold often disarrange reproduction. We do not know the time, 

 manner, nor season of gestation of the merycoidodonts, nor the number of young at a birth, nor their 

 habits at and after birth. Inclement weather, however, especially cold, at time of birth often results 

 in impoverished females, and a weakened condition of the herd is favorable for the increase of 

 carnivores, especially cats and dogs. It seems reasonably certain that temperature, rather than 

 humidity or aridity, plays a role of primary importance in delimiting areas where reproduction can 

 occur. 



Periodic flooding of areas, as happened during and after the Oligocene, may well have caught 

 and engulfed many animals. Likewise prairie and forest fires caused by lightning probably accounted 

 for the death of many of the young and old, in general the weaker and more slowly moving indi- 

 viduals. Either of these phenomena could cause a marked numerical diminution in certain areas. 



Overcrowding, by the increase of numbers of individuals in a restricted region, greatly disturbs 

 the balance of nature by altering the character and the amount of the food supply, and may even 

 change the water supply of the area. The small browsing animals, such as some of the early 

 merycoidodonts, perhaps contributed to deforestation by the destruction of young trees. In the 

 Oligocene, oreodonts and horses ranged widely in large numbers. It is reasonable to assume that 

 they helped to cause the extinction of some of the larger quadrupeds, and their own extermination 

 may have been aided by this overstocking of a given area. 



Too close inbreeding results in an increase of males, and the final outcome is often too few 

 adults to protect the young or adults without the physical vigor requisite to combat predatory 

 animals and stress climates. 



Darwin has said that the keenest competitors are the animals of most nearly similar feeding 

 habits, and this competition would rapidly increase with the numerical growth of individuals — a con- 

 dition that we know occurred in the Oligocene. Likewise, as altitudes decreased and lowlands 

 widened, the higher-level faunas were obliged to migrate or to adapt themselves for competition 

 with the lowland faunae, which had the advantage. 



Another form of competition was caused by physiographic changes whereby foreign (new) faunas 

 were introduced. This took place in nearly all of the Cenozoic epochs. It is almost always the 

 indigenous fauna which gives way before the immigrants, as witness the record of Australia. 



As the aridity increased, moist areas became more and more sought after, and there was a 

 greater concentration of faunas at or near such areas. Many of them were probably infested with 

 insects carrying protozoan diseases. More than six thousand species of insects are known from 

 American Cenozoic strata, and some of the forms have changed practically not at all from then to 

 the present. Until modern times, the maximum expansion of insect development seems to have 

 been in the Miocene. 



In the past, as today, certain areas were probably rendered uninhabitable by insect pests, such 



