594 GEOGRAPHICAL DISTRIBUTION 



In the case of locomotion, certain mountain ranges offer 

 effective barriers to certain mammals, the physical difificulties 

 being impassable. Again, a comparatively narrow strait 

 of water may act as an effective barrier to the great majority 

 of mammals. 



As regards food, the whole mammalian class is either 

 directly or indirectly dependent upon vegetable food and 

 the great determining factor in the distribution of plants is 

 temperature. It is probable that the direct effect of tem- 

 perature upon mammals is not very potent, as their hairy 

 covering with its possible variations allows of great latitude, 

 but the indirect effect through plants is very marked. Thus 

 many mountain ranges act as barriers more by virtue of their 

 great altitude than by mechanical difificulties, and ranges 

 parallel to isothermals are more effective than those in other 

 directions. Were there no other physical elements of 

 diversity than temperature, it is probable that the herbivorous 

 mammals would be evenly distributed in zones, according to 

 the isothermals or lines of equal temperature. 



Deserts may act, through absence of food and water, as 

 effective barriers, as, for example, in the case of the Sahara. 



The difficulties are multiplied when we recollect that 

 these factors of water- isolation, rock-isolation, and sand- 

 isolation are, like all physical phenomena only transitory, and 

 therefore act only for certain periods The present distri- 

 bution of mammals cannot be satisfactorily explained by an 

 appeal to the present isolative agencies, just as the present 

 environmental factors of an organism will not account for 

 its structure. In other words, the fauna of a given area 

 is determined, firstly, by its past physical history and, 

 secondly, by its present physical condition. Hence we 

 must, in dealing with the characteristic fauna of the great 

 realms, take into consideration their past as well as their 

 present. 



Throughout the Triassic and Jurassic the reptiles were 

 the dominant group, and certain of these, the Anomodontia 

 (with heterodont teeth), appear to be closely allied to the 

 amphibio-reptilian-like ancestors of the mammals. It i^ in 

 the higher strata of the Triassic that the Allotheria {Froto- 

 therid) first make their appearance, together with certain 

 types which may be Polyprotodontia {Metatheria). The 



