878 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Recent discoveries (Bruce, 1905.1) indicate that 

 immunity from disease has been one of the most potent 

 causes of the adaptation of animals to their environ- 

 ment, and that, conversely, nonimmunity has probably 

 been one of the potent causes of their diminution and 

 extinction. T. H. Morgan (1903.1) includes the 

 phenomena of immunity among the adaptive processes. 

 He states as his personal opinion, however, that 

 certain of these phenomena could not be due to selec- 

 tive processes. Similarly Leo Loeb (1905.1) believes 

 that acquired immunity can not invariably be ex- 

 plained as an adaptive phenomenon. 



Variations in immunity. — In Africa certain diseases 

 are fatal to both wild and domesticated animals; 

 others are fatal to domesticated animals but not to 

 wild animals. Some animals succumb to certain 

 diseases through successive generations; others; 

 especially "natives," acquire immunity in the second 

 generation. Still more remarkable is the fact that 

 both wild and domesticated immunes may act as 

 reservoirs of disease organisms which, through flies or 

 ticks, may be carried to nonimmunes. The wild 

 ruminants of Africa among the Bovidae especially — 

 the buft'alo {Bos {Bubalus) cajfer), the kudu {Strep- 

 siceros Tcudu), the wildebeest (ConnocJiaetes) — carry 

 about in the fluid portion of their blood, without 

 themselves suft'ering any harm, certain protozoan 

 trypanosomes which are fatal when borne by flies to 

 domesticated horses (Equidae), dogs (Canidae), and 

 cattle (Bovidae). 



Thus causes favorable either to the genesis of disease 

 organisms or to the acquirement of immunity or to the 

 propagation and distribution of flies and ticks become 

 matters of prime interest in relation to extinction. 



Origin of immunity as a "unit" character. — For the 

 student of .extinction an important point to note in 

 connection with "horse sickness" is that although 

 means of artificial immunity are thus far undiscovered 

 degrees of immunity and of natural immunity some- 

 times occur. Such variations in respect to immunity 

 would in a state of natm-e lead to the survival or grad- 

 ual selection of immune forms and the consequent 

 production of immune races. 



BULK NOT INHERENTLY INADAPTIVE 



Bulk supposed to he inadaptive. — There is a wide- 

 spread belief or tradition that bulky animals have 

 tended to disappear first — that bulk is in itself in- 

 adaptive. Thus Owen, although as late as 1877 

 (1877.1, pp. ix-x) disposed to attribute the extinction 

 of the large mammals of Australia to the agency of 

 man, advanced the theory that bulky size may be a 

 disadvantage under changed conditions. He writes: 



In proportion to the bulk of a species is the difficulty of the 

 contest which, as a living organized whole, the individual of 

 such species has to maintain against the surrounding agencies 



that are ever tending to dissolve the vital bond and subjugate 

 the living matter to the ordihary chemical and physical forces. 

 Any changes, therefore, in such e.xternal conditions as a species 

 may have been originally adapted to exist in wiU mihtate 

 against that existence in a degree proportionate, perhaps in a 

 geometrical ratio, to the bulk of the species. If a dry season 

 be gradually prolonged, the large mammal will suffer from the 

 drought sooner than the small one; if any alteration of climate 

 affect the quantity' of vegetable food, the bulky herbivore will 

 first feel the effects of stinted nourishment. * * * The 

 actual presence, therefore, of small species of animals in coun- 

 tries where larger species of the same natural families formerly 

 existed is not the consequence of any gradual diminution of the 

 size of such species but is the result of circumstances which 

 may be illustrated by the fable of the "oak and the reed"; 

 the smaller and feebler animals have bent, as it were, and 

 accommodated themselves to changes which have destroyed 

 the larger species. 



Morris observes (1895.1, p. 254): 



One tendency, which has particularly manifested itself in 

 herbivorous animals, has frequently led directly to their de- 

 struction. This is the tendency to increase in size through the 

 double influence of abundance of food and little waste of tissue 

 through exertion. In the sluggish grass eaters, dwelling on plains 

 covered with rich herbage, or leaf and twig eaters in tropical 

 forests, the nutritive agencies are in excess of those of waste, 

 and these animals seem always to have tended to an increase 

 in size, until those of least exertion and greatest powers of 

 obtaining food became enormous in dimensions. An example 

 of the same kind among the Carnivora is the Greenland whale, 

 which, while feeding on minute forms, obtains them in enor- 

 mous quantities with little muscular exertion and has in con- 

 sequence become of extraordinary dimensions. 



Statistics as to handicap of hulk. — This general 

 opinion as to the fatality of bulk in mammals is open 

 to question. When we examine the matter statistically 

 we find that a far larger number of families of small 

 mammals have become extinct than of large mammals. 

 The only families of bulky land mammals that have 

 become entirely extinct since the beginning of Tertiary 

 time are the following : 



Families of bulky land mammals that have become extinct since 

 Cretaceous time 



Coryphodonts 



Uintatheres 



Titanotheres 



Py rotheres 



Lophiodonts 



Barytheres 



Arsinoitheres 



Elotheres 



Amynodonts 



Chalicotheres 



Diprotodonts 



Giant sloths (3 



families) . 

 Glyptodonts 



Toxodonts 



Lower Eocene 



Upper Eocene 



Lower Oligocene.. 



Eocene 



Upper Eocene 



Upper Eocene 



Lower 01igocene__ 



Lower Miocene 



Lower Oligocene— 



Lower Pliocene 



Lower Pleistocene 

 Pleistocene 



Pleistocene 



Pleistocene 



Holarctica. 



North America, Asia. 



Holarctica. 



South America. 



Eurasia. 



North Africa. 



North Africa. 



North America. 



North America. 



Eurasia. 



Australia. 



North and South 



Anaerica. 

 North and South 



America. 

 South America. 



