Page:The Habitat of the Eurypterida.djvu/79

 sea as to have affected the fauna very markedly, but it will be noted that marine types must predominate in the fauna of the Baltic, as a whole, since for none of the phyla are the numbers much greater than they are at Kiel, where the fauna is wholly marine.

Perhaps the most significant fact brought out, is that the marine forms which are found in the Baltic, though they may be dwarfed or otherwise modified, are not different specifically from the marine forms found along the coasts of Great Britain, nor do the fresh-water forms differ from those found in the rivers emptying into the Baltic, or those in the neighboring fresh-water bodies. Thus it is established that in a brackish-water body of the nature of the Baltic the fauna is due to the mingling of modified marine and of modified fresh-water, that is, river forms. Only the more euryhaline marine species survive and these may in a given estuary give rise to a fauna which we may designate as a "brackish-water fauna." It will consist of forms derived in the manner just described, and these forms may become adapted to the peculiar temperature and salinity conditions prevailing in the given estuary. Thus, new mutations, varieties, and occasionally species may arise, but seldom a new genus and never a whole class of organisms. It is only the smaller taxonomic divisions which are affected. Furthermore, a "brackish-water fauna" in any estuary is always ephemeral, for the estuary is of short duration, geologically speaking.

The Severn Estuary. While the Baltic serves to show on a large scale what happens to a marine fauna which is gradually subjected to fresher and fresher water until it passes through brackish conditions to entirely fresh ones, there is another type of brackish water, the estuary, which is often said to have a fauna of its own. An estuary may be defined as the drowned lower portion of a river in which twice daily there is a change in the water from fresh to marine and back again as the tide comes in and goes out. On account of the tidal scour and thorough mixing of the marine and the inflowing river water, the brackish portion will not be very large. The Severn, on the west coast of England, is a very typical estuary, having the long, slowly broadening form toward the sea. There are a number of tributaries with their respective estuaries, so that on the whole the Severn may be considered characteristic. It is well known that muds are the dominant sediments, not only in the main tidal channel far out to sea, but also in all of the tributary channels. Professor W. J. Sollas has made a careful study of these muds in order to determine