1024 REPORT OF THE HARVARD AFRICAN EXPEDITION 
Species is in effect an island which received its complement of species at the time 
when it separated from its original mainland, the parent stock. Since that time 
it may possibly have received additions to its fauna by accidental means but in 
most cases such accidents seem not to have occurred. 
Upon the host island the factor of isolation has been at work and thus it 
follows that in general the parasite species upon widely different groups of birds 
or mammals are quite different, while those upon closely related hosts are very 
similar or even identical. It follows also that in general two or more very closely 
related species of parasite do not occur upon the same animal. This is quite in 
accord with the generally accepted dictum of students of distribution which 
holds that two very closely related species are usually found not in exactly the 
Same region but in neighboring regions. Upon the very limited area represented 
by the body of an animal, with its ecologically uniform conditions, it is difficult 
to see how two closely related species could arise, the factor of isolation being 
entirely non-operative. 
Nevertheless, there are a few cases where two or even more quite closely 
related species of parasites occur upon the same host species. The most remark- 
able of such cases that is known to me is that represented by the material at 
hand from the Procaviidae, specifically that in the bottle received from Dr. 
Bequaert. Here we have six species, some of which at least are quite closely related, 
occurring together upon a single host individual. Furthermore these are but a 
part of the species of the same group that occur upon the Procaviidae and as 
far as the present evidence goes these species are more or less generally and 
promiscuously distributed over the members of the family. It would seem that 
such a case merits a considerable degree of attention. 
I shall not here enter into any speculation concerning the possible reasons for 
the origin of a condition that departs so widely from the normal. I shall, how- 
ever, figure in detail and discuss the species involved in order to present the data 
concerning the case. 
TRICHODECTIDAE 
All of the Mallophaga in question belong to the family Trichodectidae, the 
members of which are distinguished from all other Mallophaga by a number of 
characteristics, but most obviously by the possession of one-clawed tarsi. All of 
the members of the family occur upon mammals, between seventy and eighty 
species being known. 
No careful study of the entire family has ever been made. Consequently, 
although it is possible to recognize groups of species, it is not possible at present 
to indicate generic groupings with any feeling of security. Certain genera have 
indeed been segregated from T'richodectes, but on very unconvincing grounds. 
Two genera, however, T'richophilopterus and Eurytrichodectes, are unquestionably 
valid. The latter of these includes a species that comes within the range of our 
attention here. 
Because of the existing conditions, which demand a complete review of the 
Trichodectidae before the naming of genera is undertaken, I shall not attempt to 
