208 THREE CRUISES OF THE "BLAKE." 



first deep-sea expedition of the '• Bibb," through the transpor- 

 tation of their pelagic larvae by currents to more and more distant 

 regions ; much as we account for the extension of certain deep- 

 sea faunae, into adjoining geographical districts, as in the case of 

 the northern extension of many Florida species far towards the 

 coast of New Ens^land. 



. It seems difficidt for us to speculate on the origin of the 

 pelagic fauna. Going back to the earliest fossiliferous periods, 

 when the marine fauna was made up of pteropods, gasteropods, 

 sponges, crinoids, graptolites, brachiopods, and crustaceans, we 

 have animal types whose development in these early days must 

 have been similar to that of their recent allies. Their youngest 

 stages must ah'eady at that time, as are their representatives in 

 our day, have associated with true pelagic types as part of the 

 littoral pelagic fauna of the period. There is no reason why 

 the true pelagic types of those times should not hold to the 

 free-swimming embryo of the earlier marine faunae the same 

 relations which they hold to those of our own. It seems, there- 

 fore, most natural to look upon the pelagic fauna of to-day and 

 that of former geological periods as made up of embryonic types 

 removed from the influences necessary for their fuU develop- 

 ment, and which have remained thus permanently in embryonic 

 stages, even after a time reproducing themselves, as other larval 

 forms are now capable of doing. But to consider that the lit- 

 toral forms were developed from pelagic types, as has been 

 suggested by Moseley, does not seem to be warranted by the 

 embryological history of marine invertebrates. 



Associated with the pelagic animals we find in all latitudes 

 minute algae, covering immense stretches of sea, often in suffi- 

 cient quantities to discolor the water. The black and white 

 water of the arctic, the so-called feeding-ground of whales, is 

 mainly made up of diatoms, with which are also found pelagic 

 animals. The color of the Red Sea is due to a minute alga, 

 forming huge patches of a blood-red tint. A similar phenome- 

 non is described by Darwin on the coasts of Chili and of Peru, 

 where I have frequently seen it myself. In the Gidf of Mexico 

 a pelagic alga, identical, probably, with that of the Red Sea ( Tri- 

 chodesmiwn erythroeum) (Fig. 133), is seen on calm days, in 



