594 GEOGRAPHICAL DISTRIBUTION 
In the case of locomotion, certain mountain ranges offer 
effective barriers to certain mammals, the physical difficulties 
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 difficulties, 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 is in 
the higher strata of the Triassic that the 4/otheria (Proto- 
theria) first make their appearance, together with certain 
types which may be Polyprotodontia (Metatheria). The 
