870 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



proportional numbers of almost all classes of animals and plants 

 and probably prove fatal to the existence of many which would 

 otherwise thrive there; while, on the contrary, the same occur- 

 rences can scarcely fail to be favorable to certain species which, 

 if deprived of such aid, might not maintain their ground. 



Insect barriers. — We may first consider the influence 

 of the introduction into liabitual feeding grounds of 

 various forms of insect life which render tliese grounds 

 practically uninhabitable and either kill or drive the 

 animals out. Thus Darwin, as quoted by Wallace 

 (1889.1, p. 19), observes: 



In several parts of the world insects determine the existence of 

 cattle. Perhaps Paraguay offers the most curious instance of 

 this; for here neither cattle nor horses nor dogs have ever 

 run wild, though they swarm southward and northward in a 

 feral state; and Azara and Rengger have shown that this is 

 caused by the greater numbers, in Paraguaj', of a certain fly 

 which lays its eggs in the navels of these animals when first 

 born. The increase of these flies, numerous as they are, must 

 be habitually checked by some means, probablj' bj' other 

 parasitic insects. Hence, if certain insectivorous birds were 

 to decrease in Paraguay, the parasitic insects would probably 

 increase; and this would lessen the number of the navel-fre- 

 quenting flies; then cattle and horses would become feral, and 

 this would greatly alter (as indeed I have observed in parts of 

 South America) the vegetation; this again would largely afi'ect 

 the insects, and this, as we have just seen in Staffordshire, the 

 insectivorous birds, and so onward in ever-increasing circles of 

 complexit}'. 



Insects and infection. — As noted above, the most 

 striking advance toward a complete theory of natural 

 extinction has come from recent discoveries regarding 

 the real nature of animal diseases and how they are 

 communicated. Only recently have we come thor- 

 oughly to realize, first, that insects are the most 

 active means of introducing and spreading fatal dis- 

 eases over great geographic areas and on a vast scale; 

 second, that certain immune mammals become the 

 bearers and disseminators of these diseases. 



Aflalo in his paper "The beasts that perish"** 

 has discussed many of the various causes of exter- 

 mination and gives disease a prominent place. 

 Among the Carnivora there are the nonepidemic dis- 

 eases, such as distemper, affecting dogs, foxes, wolves, 

 cats, and other wild felines. The more rare and 

 sporadic epidemics claim victims among the Carni- 

 vora wholesale. The prevalence of rabies among 

 foxes was observed on the continent from 1830 to 1838 

 in Switzerland, also in Wiirttemberg and Baden. 



Tides in Africa. — Roosevelt (1910.1) dwells on the 

 extent to which African wild mammals are persecuted 

 and infested with ticks, to which, however, they seem 

 to -have become so habituated that they dread them 

 much less than the biting flies; the ticks, even where 

 they do not introduce disease germs, are very weakening 

 as bloodsuckers. Many birds devote themselves to 

 little else than the picking off and eating of the ticks 

 and fleas infesting the mammals; if these birds were 

 killed off the mammals would suffer far more. 



68 The original article has not been accessible to the author. 



Johnston notes (1910.1) that certain types of heron 

 (egret) are perpetually snapping at tsetse and other 

 flies which settle on oxen or game and probably 

 destroy a considerable proportion of these disease- 

 carrying insects. 



Ticks as rapid spreaders of disease among domestic 

 ruminants. — Piroplasma parvum is a protozoan which, 

 imlike the trypanosome, invades the blood corpuscle. 

 It is malignant with cattle along the greater part of the 

 east coast of Africa, causing what is known as east- 

 coast fever. The infection is usually transmitted by 

 ticks, most frequently by the brown tick, Rhipi- 

 cepJialus appendiculatus , also by R. (OerotJierium) 

 simus. Migrating or trekking cattle may carry the 

 ticks many miles a day and thus spread the disease 

 rapidly over a wide area. The larva creeps on an 

 infected animal, sucks some of its blood, drops off, lies 

 among the roots of the grass, and passes its first molt, 

 becoming a nympha, then an imago, in either of which 

 stages it may infect a healthy animal by creeping 

 from the grass. The tick is very hardy and may 

 survive with its infection for a year, but after a year 

 or 15 months the infected ticks are all dead, so that 

 healthy cattle may reenter the field without risk. It 

 takes two years to starve the ticks out of a country by 

 removing the cattle. This method was employed in 

 the Wichita National Bison Preserve, the bison (Bison 

 americanus) being very susceptible to "red-water 

 fever." If this tick had been introduced centuries 

 ago among the bison it might have exterminated them. 



^Yide geographic distribution. — The geographic dis- 

 tribution of the species of Piroplasma is very wide; 

 first discovered in North America, it is now epidemic 

 throughout most of South Africa. Piroplasma bigemi- 

 num similarly causes the "Texas" or "red-water" 

 fever of our Southern States. It is conveyed by a 

 tick. The germs are latent, and the blood of an animal 

 that has recovered from Texas fever remains infec- 

 tive. Thus apparently healthy cattle may infect 

 imported susceptible cattle. Such latency has a sig- 

 nificant bearing upon the theory that natural extinc- 

 tion may be caused by similar germs. Although they 

 acquire immunity, the domesticated native Bovidae 

 act as reservoirs of the disease, in contrast to the 

 tsetse-fly disease, in which the wild Bovidae act as 

 reservoirs. The further fact that native cattle may 

 become immune has an important theoretical bearing 

 on the natural origin of immunity to the tsetse-fly 

 and other diseases among the wild Bovidae and wild 

 Equidae. 



TicTcs among Equidae. — The biliary fever of domes- 

 ticated Equidae (horses, mules, donkeys) is conveyed 

 by a corpuscle parasite (Piroplasma equi), which is 

 spread by the red tick (RhipicepJialus evartsi), the 

 infection being taken in the nymphal and transferred 

 in the adult stage. The native South African horses, 

 like cattle that are exposed to Texas fever, become 



