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the long run, it may be supposed, displace the ones retarded by a per- 
sistence of useless structures. 
In this way I believe we may account, consistently with the best 
founded views on the action of natural selection, for the origin and de- 
velopment of the parasitic habit in Sarcoptidse, and for those peculiari- 
ties of structure which make it a family. 
In its ensemble of characters it is simply one of many illustrations of 
the effect of the struggle for existence and the action of natural selec- 
tion. It has come into existence because its members obtained some 
advantage over certain other mites by taking on the parasitic habit. 
This is a scant treatment of the subject, but I take it that I am not 
expected to present at this time an extended and detailed argument. 
I could have dwelt on the evidence for the degeneration of mites; could 
have given evidence for crustacean affinities of Pentastomum, not per- 
haps open to objections which can be made to relations with the Vermes; 
could have given evidence for the relations of Phytoptus with the spin- 
ning mites and against relations with Demodex. I have not neglected 
these matters with a purpose to obstruct a view of the truth. The 
book lies open; he who will can examine the record for himself. But 
it is my opinion that an examination of the subject along the lines here 
pointed out will satisfy the candid mind that mites are degraded Arach- 
nida; that Sarcoptide are degraded mites, and are not the lowest 
in rank of the order; that their parasitic habit has been recently as- 
sumed, and that their immediate ancestors were free-living mites. 
ORIGIN AND DEVELOPMENT OF THE PARASITIC HABIT IN 
MALLOPHAGA AND PEDICULIDA.* 
By HERBERT OSBORN, Ames, Iowa. 
Inasmuch as the Mallophaga and Pediculide are limited to the warm- 
blooded vertebrates in their host relations, it is not to be wondered at 
that they present many points of correspondence, and that notwithstand- 
ing the great difference in their fundamental structure there should be a 
number of very strong cases of parallelism in the modification of struc- 
ture resulting from the similar conditions under which they live. On 
this account it is convenient to discuss them jointly and to compare 
those organs which have been most responsive to this environment. 
The group Mallophaga contains an assemblage of insects very clearly 
defined and distinctly separated from any of the related insects, so 
isolated, in fact, that their position has been the subject of no little 
discussion. A review of this discussion is not contemplated here, and 
would be out of place, except so far as it might bring out the structural 
modifications to be met with. So far as the affinities of the group are 
* Read before Section F., A. A. A. S., at the Washington meeting, August, 1891, 
