CAUSES OF EXTINCTION 273 



as the tee-tee fly in Africa, or the ticks among horses, even those that do not carry disease being very 

 weakening, because of their bloodsucking habit; or certain parasitic larvx which enter the skull 

 sinuses might have the same effect. 



Infectious diseases, introduced by insects, are prevalent today among the gregarious Herbivora 

 and probably were so in the geologic past. Insects can spread diseases over a great territory and on a 

 huge scale While perhaps not in themselves causing extinction, diseases could easily diminish the 

 numbers of quadrupeds to the point where extinction would be the inevitable result. Evidence of 

 disease among the oreodonts is, however, extremely rare. 



_ Immunity is one of the greatest of animal adaptations to environment, but immunity is not 

 attained until after many individuals have perished. Animals that are immune, that is, immune to 

 the effects but not to the presence of poisons or internal parasites, often act as reservoirs from which 

 insects replenish their supplies of the poisons or of the parasites which may be, in turn, passed on to 

 the non-immune faunas. 



It is conceivable that insects, like the locusts, may have destroyed the vegetation over consider- 

 able areas, perhaps periodically, thus causing much hardship to nearly all classes of animals, as well 

 as to the plants themselves. ' 



Migrating animals could well have carried insects or their larva; and spread them widely among 

 the native fauna, thus producing epidemics among gregarious forms. Epidemics must have wiped 

 out large numbers of quadrupeds in certain areas in the past and must be considered as at least an 

 occasional means of bringing about extinction. 



Carnivores undoubtedly took some toll of the smaller merycoidodonts, especially as the latter 

 had no adequate means of defense or of flight and as their ability to resist their enemies probably lay 

 in herd characteristics By analogy with modern artiodactyls, we presume that they lived in bands 

 of rather limited numbers. They had keen senses of smell and of sight, which were of great aid in 

 preserving them from their enemies, but their general life habits were based on a complex of reflexes 

 with very little volitional activity on the part of the individual. They were vulnerable to attack in 

 theopen country but probably much more so along the streams and around water holes, though here 

 again we cannot be sure that they required recourse to open water for their sustenance. Durin* the 

 stress periods the groups probably lost vigor, and, as a corollary, the carnivores waxed stronger? and 

 doubtless many a meryco.dodont, both of young and old, fell a victim. Today large numbers of 

 young peccaries are killed by predatory animals. 



Some years ago I made a quantitative study of several hundreds of specimens of Merycoidodon 

 and Eforeodon for the purpose of determining the relative age of the individuals at death. There is 

 a remarkable similarity in the results between the West Coast and the Great Plains genera, widely 

 separated both geographically and in geologic time, as well perhaps as in environmental conditions 

 ri, t J ^^y.^ese studies showed that 65.7 per cent in the Great Plains and 65.6 per cent" in 

 the John Day Basin died in the prime of life. In the former area 20.9 per cent died before maturity 

 and in the latter but 13.2 per cent, indicating a higher rate of infant mortality in the Great Plains 

 Merycoidodon. Of those that reached a ripe old age, only 3.3 per cent are recorded from the 

 Great Plains and 8.8 per cent in the John Day. Between maturity and the beginning of old age 

 10 per cent died in the former area and 12.2 per cent in the latter. Hence it seems that over 

 95 per cent of the Great Plains Merycoidodon did not reach old age, while in the John Day Basin 

 somewhat more isolated perhaps, about 85 per cent of the Eforeodon fauna died before old age' 

 1 do not mean that this ratio applies to all the genera of merycoidodonts or to the Oligocene- 

 Miocene mammalian fauns in general, but the results indicate that in these two fossil groups very 

 few individuals survived to old age. The figures for the Great Plains Merycoidodon were compiled 

 from specimens not in any one quarry but from four or five States and from all of the horizons in 



Ki, r^r ", f T 0U , ncL C amiv °res were fairly numerous and were well differentiated in 

 both the Great Plains and John Day Basin areas. 



OH I'' th l kst fC T ^ arS the d V St St0rms b the s °- caI1 ed "Dust Bowl" area in and around 

 Uklanoma have worked great hardship on the domestic stock and on the wild fauna. The effect has 



