SUMMARY OF RESULTS. 
1451 ) 
shore and shallow- water origin. In polar waters a marked peculiarity of the tow-net 
gatherings is the almost total absence of pelagic larvae belonging to benthonic 
organisms, and we know that many of the Echinoderms and other shallow-water 
animals of the Arctic and Antarctic regions are furnished with pouches in which 
the young are reared d the same appears to be true of the animals living about 
and deeper than the mud-line in all parts of the world. In temperate zones 
where there is a wide range in annual temperature the pelagic larvae of benthonic 
animals appear only in the spring or summer seasons ; in the tropics they are 
present at all times in the surface waters. If there were once a nearly universal 
climate over the whole ocean, we may suppose that the same species of benthonic 
animals were nearly everywhere present in the shallow-water zones. When coolino- Marine 
/ ^ ® AND Fresh-Water 
at the poles set in, those animals with pelagic larvae would be killed out or be Faunas. 
forced to migrate towards the warmer tropics. By being able to limit the reproductive 
process to the summer season, some of these organisms with free-swimming larvae have 
been able to live on in the temperate regions, but in the tropical and coral-reef regions we 
have the remnants of a once universally distributed shallow- water fauna. With the 
disappearance of this shallow-water fauna from the polar regions its place would be 
occupied by the organisms from the deeper mud-line, very few of which possess 
pelagic larvae. In this way we may account for the identity or similarity between 
the polar marine faunas and floras, the great abundance of individuals and the 
relatively few species in the polar areas when compared with the tropical area, as well 
as the greater likeness of the shallow-water polar animals to deep-sea species. In like 
manner we may account for the disappearance of coral reefs from the west coasts of 
Africa and America and off Cape Guardafui, owing to the wide range of temperature in 
these positions from upwelling of cold deep water. Organisms derived recently from the 
mud-line animals here too occupied the shallow waters, and hence more closely 
resemble polar faunas than any other fauna within the tropics. From the general 
character of fresh-water species, and from the almost complete absence of free-swimming 
larvse, we may suppose that the fresh-water fauna has also been chiefly derived from 
mud-line animals which ascended from the mouths of great rivers and from estuaries. 
The general similarity, and, in many cases, the identity of species of marine 
organisms in the Arctic and Antarctic regions, is a very remarkable fact, but probably 
not a more remarkable fact than that in the tropical regions there should be among 
benthonic animals hardly a marine species common to the east and west coasts of the 
continent of Africa, if we exclude some brackish-water and deep-sea species. The 
<^reater annual range and higher temperature, as well as the greater variety in the 
other conditions which obtain on either side of Africa, compared with those that obtain 
^ See Narr. Chall. Exp., vol. i. x>. 3V9. 
