874 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



true of the heart-water disease of cattle, goats, and 

 sheep (Bovidae), which is similar in distribution to the 

 horse sickness and is caiTied by the bont tick {Am- 

 hlyomma hehraeum), in that it dies out on the high 

 veldt. The "catarrhal fever" of sheep has a distribu- 

 tion ill South Africa similar to that of horse sickness 

 and is probably carried by means of the same night- 

 feeding insect. The infection causing horse sickness 

 is not carried into the high country nor during the 

 dry season; the parasite is unknown and is believed 

 to be ultramicroscopic ; it is believed to be carried in 

 the blood, because the one-thousandth part of a single 

 drop of blood injected under the skin of a healthy 

 animal will cause death; some horses require a larger 

 dose than others, indicating fluctuations in power of 

 resistance or immunity. Unlike the foregoing dis- 

 eases it is not endemic or permanent but occurs in 

 epidemics at intervals of 10 to 20 years. Its geographic 

 distribution in South Africa is very wide, including 

 Natal, Zululand, the greater part of Rhodesia, 

 Bechuanaland, and Portuguese East Africa. Horses 

 placed in fly-proof shelters, even in exceedingly 

 unhealthy places, in no case incur the disease. The 

 particular fly or insect carrier is stiU unknown. As 

 in several of the foregoing diseases the infective power 

 of the blood persists for years. 



Summary as to natural extinction iy epidemics. — To 

 summarize these remarkable observations, which we 

 owe to the labors of Lewis, Koch, Theiler, Kilborne, 

 Smith, Watkins-Pitchford, Lounsbury, Salmon, Cur- 

 tice, Stiles, Hassall, Taylor, and many others, we 

 undoubtedly have an agency that must be considered 

 an occasional if not a frequent cause of the extinction 

 of quadrupeds in the past. 



It will be noted (1) that in the case of the tsetse-fly 

 disease the wild ruminants are the permanent though 

 unharmed carriers to domestic animals of the infective 

 protozoan; (2) that, on the contrary, in the Texas 

 fever, or "red-water fever" the native immune 

 Bovidae are the permanent carriers of the disease 

 organism to the wild Bovidae (bison); (3) that the 

 rinderpest now appears to be in an early stage of its 

 history as a disease in which neither domesticated nor 

 wild Bovidae have become naturally immune, and all 

 the Bovidae act as reservoirs; (4) that in the east- 

 coast fever the infective ticks survive for a year and 

 the permanent carriers of the infective organism are 

 not yet discovered; (5) that in the biliary fever of 

 domesticated horses the recovered equines act as 

 reservoirs; (6) similarly again, that in the "horse sick- 

 ness" of South Africa the infective power of the blood 

 in a recovered animal persists for years. 



Thus in these modern cases we have all the theo- 

 retical conditions favorable to the wide distribution of 

 insect-borne diseases which in past times may have 

 attacked various types of quadrupeds and resulted in 

 extermination before natural immunity was acquired. 



The relation of birds to the life of insects that attack 

 mammals, and thus to the existence of mammals 

 themselves, has already been mentioned. The closest 

 relation of birds to the protection of mammal life is 

 that of destroyers of the insects which infest and 

 infect mammals. 



The bird that is most notably destructive to mam- 

 mals is said to be the kea, the New Zealand parrot 

 (WaUace, 1889.1, p. 75). The sheep-killing habit of 

 the kea has recently been declared mythical. W. B. 

 Benham (1906.1; 1907.1), however, asserts that the kea 

 causes great mortality among the sheep of the high 

 mountainous country of the South Island; in other 

 parts of New Zealand it is absent. The kea perches 

 on the back of the sheep and soon eats a hole into the 

 abdomen. It is not known whether this is for the 

 purpose of getting the blood or of eating the kidney 

 fat, as was formerly supposed. The sheep are often 

 found with a hole so large that the entrails exude and 

 the animal has to be killed. The kea does a great deal 

 of its work at night, and for this reason a great many 

 people have not been able to observe it and have 

 doubted the truth of the statements. Other birds are 

 known to have turned from herbivorous to carnivorous 

 habits. 



MAMMALS 



Forms oj competition. — From consideration of the 

 struggle of animals with their physical environment 

 and with their living plant, insect, and bird environ- 

 ment, we now pass to consideration of the more 

 intimate struggle with other mammals. It is impor- 

 tant to note that the struggle in this intermammalian 

 competition is always intensified during periods of 

 geographic, climatic, and biotic change. This struggle 

 naturally presents a variety of phases, such as com- 

 petition of lower and higher types of mammals, 

 struggle between Herbivora and Carnivora, struggle 

 between Herbivora and Herbivora, competition be- 

 tween resident and newly introduced forms, and com- 

 petition between less and more adaptive types. 



Even in comparison with the supposed climatic, 

 plant, and insect agencies, the competition between 

 lower and higher types, or between less and more 

 adaptive types, or between slowly producing and rapidly 

 producing types has been the chief direct agency of 

 extinction, because it has worked more widely and 

 over longer periods of time. 



During the introduction of new types in the Ter- 

 tiaiy period in North America, we witness more than 

 once the apparently unchecked multiplication of cer- 

 tain local or native mammals; the repeated intro- 

 duction or migration of new mammals, either 

 singly or in waves; and the slow or rapid sequent 

 extinction of certain local mammals. These cycles 

 of change may be due to remote geographic conditions 

 but are frequently connected with or sequent upon 



