430 THE AMERICAN NATURALIST. (VoL. XXXIII. 
the electric theory; waves, especially sound waves; the physical 
theory of hearing and musical instruments; and valuable chapters on 
Elementary Theory of Optical Instruments, Spectroscopy, and Maxi- 
mum Efficiency of Optical Instruments. 
The Plankton of the Limfiord.! — This fiord is a tortuous channel, 
92 miles in length, which traverses the peninsula of Jutland. In 
1825 an irruption of the North Sea drove back the fresh and brackish 
water fauna of the fiord and replaced it with that of the sea. A 
slow current passes through it from the east or the west according to 
the relative levels of the North and Baltic Seas, though in recent 
years it has been predominantly from the North Sea eastward. The 
stream is shallow — 2—13 fathoms — with a few expansions in shallow 
lakes, and exhibits practically uniform conditions of temperature and 
salinity at the top and the bottom. Owing to the slight depth the 
temperature rises to 18.4° C. in the summer, and the salinity averages 
about 3 percent. The plankton of this interesting region has been 
investigated by Dr. Petersen of the Danish Biological Station. Three 
traverses of the fiord were made in 1896 and 1897, and the collections 
thus made were supplemented by a seasonal series and by others 
from the Cattegat and the Baltic. The qualitative examination, made 
-by Mr. H. Grau, was confined in the main to the diatoms and the 
Peridiniex, which constitute the bulk of the plankton. The investiga- 
tion determined that the amount of plankton per O meter in the 
shallow fiord was ro—5o times as great as that of the adjacent and 
deeper North Sea, and more than double that of the Cattegat. The 
constitution of the plankton is peculiar in that the predominant spe- 
cies are neritic rather than oceanic, and are not thus abundant in the 
North Sea, whence the fiord receives its water, nor in the Cattegat, 
into which it empties. Its diatom flora must therefore breed in the 
water in transit in response to some change in the physical-chemical 
environment, as, for example, contact with the bottom, rise in tempera- 
ture, or the addition of nitrogen by the tributary rivulets. Another 
instance of this unique phenomena — namely, the maintenance of a 
peculiar plankton at a given point in’a body of water traversed by 
a current — was discovered in the Cattegat, where an undercurrent 
from the deeper Skager Rack enters this shallower area, and there 
is developed within it, in passing, a local and peculiar diatom flora. 
The results of this work lead the author to suggest that “guide 
1 Petersen, C. G. J. Plankton Studies in the Limfjord, Rep. Danish Biol. 
Station, vol. vii, 1897. 23 pp., with 1 map and 4 tables. 1898. 
