187 



the long- run, it may bo supposed, displace the ones retarded by a per- 

 sistence of useless structures. 



In this way T believe we may account, consistently with the best 

 founded views on the actiou of natural .selection, for the origin and <le- 

 velopment of the parasitic habit in Sarcoptidie, and for those peculiari- 

 ties of structure which make it a family. 



In its ensend)le 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 

 exi)ected to present at this time an extended and detailed argument. 

 1 could have dwelt on the evidence for the degeneration of mites ; could 

 have given evidence for crustacean afHnities of Pentastomum, not per- 

 haps open to objections which can be made to relations with the Vermes; 

 could liave given evidence for the relations of Phytoptus with the spin- 

 ning mites and against relations with Demodex. T 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 Sarcoptidse 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-Uviug mites. 



ORIGIN AND DEVELOPMENT OF THE PARASITIC HABIT IN 

 MALLOPHAGA AND PEDICULIDiE.* 



By Herbert Osborn, Ames, Iowa. 



Inasmuch as the Mallophaga and PedicuUdse are limited to the warm- 

 blooded vertebrates in their host relations, it is not to be wondered at 

 that they i)resent 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 groui^ 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 Washingtou meeting, August, 1891. 



