ZOOGEOGRAPHY AND ECOLOGY OF THE LAND AND FRESH- 
WATER MOLLUSKS OF THE BELGIAN CONGO! 
The composition of the molluscan fauna of a country is the result 
of two different sets of causes. Some act at present and are directly trace- 
able to the environment, so that it takes comparatively little effort to 
analyze them in order to discover in how far they are responsible for the 
presence or absence of certain types or for changes in external form and 
behavior. Others may be called historical and are much more subtle, 
for they are essentially the environmental conditions of past geologic 
times, especially such as rendered possible the successive migrations and 
the survival of the various elements from which the present-day fauna 
evolved. In this ecological and geographical account of the mollusks of 
the Belgian Congo we have attempted to trace the influence of each of 
these two sets of causes upon the constitution of the fauna. This chapter 
should, however, be regarded as nothing more than an essay, which we 
are well aware is in many ways sketchy. Positive observations on the 
habits and distribution of tropical African mollusks are indeed so few 
that no very decisive conclusions could be drawn from them. Our chief 
aim is to suggest lines along which various problems may be further 
investigated. With this in view, we have rather liberally added refer- 
ences to the literature, preferably selecting papers containing further 
bibliographic information. 
Before entering into a discussion of the peculiarities of the Congo 
fauna, it may not be amiss to mention certain sources of possible error 
in the study of the geographical distribution of African mollusks. In 
the first place, it is evident that the Ethiopian Region as a whole, and 
the Belgian Congo in particular, have thusfar received but scant attention 
on the part of malacologists. Much of the published information is based 
upon collections gathered incidentally by laymen and therefore usually 
containing only fair-sized or particularly abundant species. The minutiz 
are still to a large extent unknown. In addition, these collections as a 
rule consist of dead specimens only and in the absence of soft parts a 
correct generic identification is sometimes impossible. 
Another point of considerable importance is the facility with which 
certain tropical mollusks may be carried over great distances by the 
agency of man.? Especially with regard to the species common to 
India. the east coast of Africa, Madagascar, and the Mascarenes, it 
1Much of the physical, geological, and botanical data on which the present ecology of Congo mol- 
lusks is based has been taken from an unpublished description of the Belgian Congo prepared by the 
Junior author for The American Museum of Natural History. 
*See Kew, H. W. 1893. ‘The dispersal of shells.’ (London), pp. 178-263. 
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