208 THREE CRUISES OF THE “BLAKE.” 
first deep-sea expedition of the “ Bibb,” through the transpor- 
tation of their pelagic larvze by currents to more and more distant 
regions; much as we account for the extension of certain deep- 
sea faune, into adjoining geographical districts, as in the case of 
the northern extension of many Florida species far towards the 
coast of New England. 
It seems difficult 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 already 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 faune 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 full 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 ht- 
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 alg, 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 dendeibed by Darwin on the coasts of Chili and of Peru, 
where I have frequently seen it myself. In the Gulf of Mexico 
a pelagic alga, identical, probably, with that of the Red Sea (Tri- 
chodesmium erythreum) (Fig. 133), is seen on calm days, in 
