160 THREE CRUISES OF THE " BLAKE." 



the tertiaries of Western France and of Egypt; the above-named 

 West Indian spatangoids and clypeastroids, as well as CceIo- 

 pleurus and Macropneustes, disappeared first from the Eastern 

 Atlantic. The past history of the ten West Indian genera al- 

 ready found in the cretaceous, and of the twenty-four genera 

 descending from the earlier tertiary, gives us but slight assist- 

 ance in determining the mode of their origin in the Caribbean 

 fauna. 



It would be most interesting to be able to make a compari- 

 son of the deep-sea Panamic fauna with that of the Caribbean, 

 and to ascertain if, in the continental and abyssal regions, at 

 the depths beyond which the effects of motion, of light, and 

 of heat cease to be prominent factors, there is as marked a dif- 

 ference in the representative species as in those of the littoral 

 fauna. 



Soon after the end of the cretaceous period the specialization 

 of the Atlantic and Indo-Pacific marine realms began. Before 

 that time the equatorial currents swej)t nearly uninterruptedly 

 round the globe, and maintained across the Indo-Pacific and 

 Atlantic the conditions which existed in the Western Atlantic 

 before the equatorial currents became deflected by the West 

 India Islands and the northern extremity of South America. 

 If the physical causes we now see at work have, as they have 

 become altered, also modified the fauna of the equatorial belt 

 district then existing, we should naturally expect to notice after 

 a long period of time the changes thus brought about. We 

 are perhaps justified in ascribing to the subdivision of this 

 equatorial belt, into an Indo-Pacific and an Atlantic district, 

 the marked changes perceptible in the character of the fauna 

 as regards the genera which date back to the late cretaceous, 

 and the changes still more marked in the genera which date 

 from the tertiary period. 



How far it is possible for us directly to follow these modifica- 

 tions, and to trace the transition of the older fauna into the 

 characteristic West Indian fauna of to-day, is another question. 

 It involves the necessity of tracing back from the triassic and 

 Jurassic periods the genera which have appeared in succes- 

 sion ; how far this is practicable I have attempted to show on 



