84 SECTIONAL ADDRESSES. 
Moreover, the interpretations of structural details must to a large 
extent depend upon the satisfactory elucidation of these problems of 
classification, so that the solutions of the two really go together. It 
is my firm conviction that the most satisfactory solution of the first, 
and therefore also of the second, of these problems will be found in the 
application of the principles of evolutional Paleontology. 
Variation in Shallow-water Fauna. 
As regards the more fundamental of the two problems, a general 
principle seems to be involved, demanding the recognition of the rela- 
tive values of faunal changes in shallower and deeper waters respec- 
tively. The faunas of the shallow seas must of necessity be subject 
to far greater degrees of physical change than those of the deeper 
waters, and, thanks to the excellent work done at the Danish Biological 
Station! in carrying out investigations on the bottom faunas of different 
Danish waters, we now know a good deal as to the extent to which 
the distribution of modern faunas is governed by physical conditions. 
Some of the more important conclusions reached by the Danish investi- 
gators may be summarised as follows :— 
1. That certain characteristic animal communities undoubtedly 
exist under certain physical conditions, and when these conditions 
remain constant even over wide areas the same community will be 
found, but each community is bounded by those physical conditions. 
2. That change in physical conditions brings about a change in the 
characteristic animal community, though certain organisms may be 
found in more than one community. 
3. The physical changes to be correlated with the change in com- 
munity are those of temperature, salinity, and clearness of the water; 
depth as depth seems to be less important than the factcrs which go 
with depth, such as temperature, amount of light, character of the sea 
bottom, and quietness of the water. Thus along a section in the 
N. Kattegat at depths varying only between 7 and 50 metres, five 
different animal communities have been recognised :— 
Community Depth Character of Bottom Temp. 
, 
1. Echinocardium Community . | 7 metres | Fine sand _ 
2. Echinocardium Turritella |12-19 metres; Dark sand with -- 
Community fine detritus 
3. Brissopsis - Turritella - Echino- | 24.5 metres | Fine sand with fine | 14° C. 
cardium Community (transi- | particles 
tion) | 
4, Brissopsis - Turritella Com- | 35 metres | Grey Kattegat Clay 13.4° C. 
munity 
5. Brissopsis-Nucula Community | 50-52 gress Light Kattegat Clay | 8°—6° C. 
1 1913, Petersen, ©. G. J Report of the Danish Biological Station. 
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