270 KEY TO TERRESTRIAL HISTORY 



ditions retreat polewards, temperate conditions tread upon 

 their heels, till a sub-tropical or even tropical climate pre- 

 vails in temperate latitudes, while the ice in the Arctic 

 regions grows thinner and finally dissolves away. If, as 

 would seem most probable, this gradual rise of temperature 

 was due to some general cause, then again we are led to 

 the conclusion that in cases where independent testimony 

 is forthcoming, homotaxis and contemporaneity prove to 

 be interchangeable terms. 



The underlying Mesozoic deposits might also be called 

 as witnesses for the defence, but we can afford to pass 

 them over and proceed at once to the examination of the 

 Carboniferous system. This is pre-eminently the system 

 of coal-fields, which are widely distributed over the tem- 

 perate regions of both the northern and southern hemi- 

 spheres, and extend far within the limits of the Arctic zone. 

 The coal of these deposits is the product of ancient 

 forests, which, as botanists assure us, flourished in a moist, 

 equable, and fairly warm climate ; and this, as we may 

 infer from the distribution of the coal, prevailed over the 

 temperate and Arctic regions of the globe. But when we 

 pass upwards from these beds into those of the overlying 

 Permian system we encounter signs of extensive glacial 

 action. In Australia, South Africa, and India deposits 

 are met with as truly glacial as those of the much 

 later Pleistocene. These Permian beds are, indeed, 

 related to the Carboniferous much in the same way 

 as the Pleistocene to the Tertiary ; and thus, unless 

 climate is a purely local phenomenon, we are once more 

 presented with evidence to suggest that homotaxy and 

 contemporaneity are equivalent terms. 



Leaving these arguments for what they are worth, and 

 they can only be regarded as establishing a prima facie 

 probability, we may turn next to another point of view, 

 suggested by the American geologist, Professor Heilprin. 



