134 A Few Words about Scavengers. [ Mareh, 
I hasten to say, however, that bivalve mollusks do not feed 
wholly and probably not inainly on decaying organic particles. 
The currents of water, alluded to above, bear also all kinds of 
microscopic plants and animals which abound in the water where 
these bivalves live. 
To what extent radiates and protozoans are represented by 
scavengers I am not now prepared to state, but reasoning from 
(Fie. 22.) STROMB (Fra. 23.) FRESH-WATER MUSSEL 
(STROMBUS PUGILIS). (UNIO COMPLANATUS). 
what we see in the other branches of the animal kingdom, we 
may at least suppose that there are true scavengers in these 
lowest branches also; and that their structure and appearance 
are perhaps as remarkable as are found in the higher forms. 
And is there not something remarkable in the general ap- 
pearance of scavengers as well as in their habits? Does not the 
hyena present a marked appearance among mammals, and the 
vulture among birds ? 
The facts about scavengers suggest many interesting ques- 
tions. Why are there scavengers at all? Especially why are 
there scavengers in localities and regions where living animal 
food is apparently in great abundance? What is their origin? 
Have they existed from the beginning of animal life on the 
globe? When did the first individuals begin to appear in the 
various groups? Is their structure the result of their habits, or 
have their habits determined their structure ? 
The facts about scavengers are well known to all naturalists ; 
and they are facts, without doubt, of deep significance. But 
am not aware that the existence of scavengers has been explained 
in accordance with the modern views in biology. 
It may possibly be replied that there is nothing specially re- 
markable in the existence of scavengers, when viewed in the 
light of the doctrines of natural selection. There is but little 
