S64 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



seasons may cause a temporary loss of food supply or 

 cause the death of the young, which in northern lati- 

 tudes are usually born in spring. The diminution or 

 loss of yoiuig from this cause might act as the first of a 

 series of destructive effects of a progressive secular 

 change. These may be summarized as follows from 

 actual zoological observations^" among the Cervidae: 

 (1) Disturbed conditions during the conjugation 

 (pairing, mating, rutting) period; (2) enfeebled 

 (through hunger) condition of females during partu- 

 rition period; (3) severe weather conditions, ice storms, 

 crusted snow, prolonged wet and sleet at time of birth; 

 (4) bulls unable to protect herds; (5) cows unable to 

 protect young from Carnivora through starved con- 

 dition, or abandoning them when attacked by wolves; 

 (6) enfeebled and unprotected condition of quadru- 

 peds favorable to increased food supply and conse- 

 quent multiplication of cursorial and other Carnivora, 

 especially Canidae and Felidae. 



These zoological observations are to a certain extent 

 borne out in paleontology by Andrew Leith Adams's 

 observation (1879.1, p. 98) of the exceptionally large 

 number of milk teeth of elephants found in certain 

 Pleistocene deposits, which appears to indicate a high 

 mortality of the young. 



Merriam (1892.1), in what he has called the "law of 

 temperature control," has directed attention to the 

 physiological effects of a lowering of temperature upon 

 diminished or increased fertility and the rate of repro- 

 duction. It is stated as follows: Temperature, by 

 controlling reproduction, predetermines the possibil- 

 ities of distribution; it fixes the limits beyond which 

 species can not pass; it defines broad transcontinental 

 barriers within which certain forms may thrive if other 

 conditions permit, but outside of which they can not 

 exist, be the other conditions never so favorable, 

 because the sexes are not fertile. 



In discussing how species are checked in their efforts 

 to overrun the earth Merriam points out that more 

 potent than geographic barriers are climatic bai'riers 

 (as observed by Humboldt), and of these temper- 

 ature is more potent than humidity. First, in 1892, 

 Merriam attempted to show (1892.1, pp. 45-46) that 

 the distribution of terrestrial animals is governed less 

 by the yearly isotherm or mean annual temperature 

 than by the total temperature during the period of 

 reproductive activity and of growth (adolescence). 

 This reproductive period in the Tropics extends over 

 many months or nearly the whole year but within the 

 Arctic Circle and summits of high mountains is of two 

 months or less duration. Later results that Merriam 

 (1894.1) obtained from extensive comparison of tem- 

 peratures and distribution justified the belief that 

 animals and plants (lower austral and tropical types 

 coming from the south) are restricted in northward 



B^ Communicated by Mr. Madison Grant, secretary of tlie Zoological Society 

 of New Yorli. 



distribution by the total quantity of heat prevailing 

 during the season of development and reproduction. 

 Conversely, animals and plants (upper austral, transi- 

 tion, and boreal types coming from the north) are 

 restricted in southward distribution by the mean tem- 

 perature of a brief period covering the hottest part 

 of the year. Thus in the transition zone boreal and 

 austral types mingle in the equable climate of the 

 Pacific coast of California, whereas they are sharply 

 sepai'ated by the inequable extremes of cold and heat 

 of the interior continental plateau. 



Lowering of temperature and diminished fertility. — 

 The favorable influence of high temperature on fer- 

 tility and reproduction is well illustrated in the early 

 age of reproduction in tropical lands and the increase 

 in the fertility of the human species toward the 

 Equator, and as low temperature is thus a barrier to 

 reproduction it is reasonable to suppose that the 

 secular lowering of temperature may have been one 

 among the causes of the extinction of animals during 

 the glacial age. Thus certain mammals, although 

 they were otherwise becoming adapted to the effects 

 of cold and were discovering new means of feeding, 

 may have become extinct through the subtle inhibi- 

 tion of fertility and the lowering of the rate of repro- 

 duction. 



Increasing moisture in Temperate Zone. — Humidity, 

 observes Merriam (1894.1) in the work cited above, 

 is a less potent factor than temperature in limiting 

 the distribution of the mammals of North America 

 [that is, in cold and temperate climates]. Thus 

 many genera adapted to restrictions of temperature 

 zones range east and west completely across the 

 American Continent, inhabiting alike the humid and 

 arid subdivisions, but no genus adapted to certain 

 restrictions of humidity ranges north and south along 

 the temperature zones. Thus, according to Merriam, 

 humidity governs the details of distribution of a few 

 species of mammals within certain temperature zones; 

 temperature establishes the great wall, and humidity 

 establishes the lesser barriers. 



This law may be understood as the direct influence 

 of temperature and humidity respectively; the indi- 

 rect or secondary influences of temperature and 

 humidity may be entirely different. Thus Merriam's 

 generalization would not apply to central Africa, 

 Central America, or other tropical countries where 

 insect and disease barriers exist which are generated 

 and favored by high temperature and increasing 

 moisture. Because of the connection between insect 

 distribution and moisture in Africa, humidity is con- 

 sidered to be a very potent factor in the distribution 

 of animals. 



Moisture in the Equatorial Zone. — We observe that 

 increased rainfall may have the following effects, 

 especially upon the Herbivora : (1 ) It may diminish the 

 supply of harder grasses to which certain quadrupeds 



