220 GEOLOGICAL DISTEIBUTIOK. 



thus far received but little attention from physiographers, must not 

 be lost sight of in this connection, and that is, a difference in depth 

 of the oceanic abyss. On the theory of the permanency of oceanic 

 and continental areas, and the strong probability that the oceanic 

 basins really represent areas of subsidence, it may be confidently 

 assumed that the floor of the ocean has been pretty steadily subsid- 

 ing, from first to last; and, further, that the continental off-flow, 

 produced by the rise and development of the land-masses, will have 

 just as steadily tended to increase the depth of water in these basins. 

 At what rate this subsidence and gradual deepening may have taken 

 place it is impossible, in the present state of science, to determine, 

 and therefore we possess no means of ascertaining what might have 

 been the difference in depth for any two widely separated periods 

 of geological time. But it is by no means improbable that the 

 depth of sea during the greater part of the Paleozoic era was very 

 much less than it is at the present time ; indeed, we are almost irre- 

 sistibly led to this conclusion by our knowledge that the water for- 

 merly occupied a much greater lateral extension, measured on our 

 present sphere, than it now does, and this, in addition, at a period 

 when the equatorial circumference of the globe was probably con- 

 siderably in excess of its present twenty-five thousand miles. With 

 only a moderately deep sea, the difficulty in accounting for a phe- 

 nomenally broad specific distribution is greatly lessened.* 



When we seek to determine at what period the existing condi- 

 tions of distribution, or approximately such, were first introduced, 

 we find that it was not until the beginning of the Tertiary, although 

 the gradual modifications leading up to this change are clearly 

 traceable in the successive periods following the Paleozoic era. Of 

 some thirty-six species of fossils described by Coquand and Bayle 

 from the Jurassic deposits of Chili, South America, no less than 

 twenty, or more than one-half, are forms which are also found in 

 the equivalent deposits of Europe, and a fair proportion of these 

 have been identified at various points on the continent of Africa, in 

 the peninsula of India, the Himalayas, and elsewhere. The Jurassic 

 Cephalopoda of Kutch, India, comprise, according to Waagen, 72 one 



* It is true that, through terrestrial absorption, the oceanic mass is under- 

 going diminution in bulk ; but it may be questioned whether the shallowing 

 produced thereby, since the beginning of the Paleozoic era, has very materially 

 affected the general depth of water. 



