OF WASHINGTON. 421 



the day, among the litter which these use for resting or breeding 

 purposes. 



In the Thysanura, which are the most generalized of all the 

 Hexapodous insects, we have in general a decided free condition, 

 and yet there is an example in the species, described by Megnin 

 under the name of Podurhippus pityriasicus, where the para 

 sitic habit has been assumed. The passage from the non-para 

 sitic to the parasitic condition is in this case easily traced, as the 

 common species of Podura and their allies live in moist situations, 

 feeding upon decaying animal matter, and the transfer to the feet 

 of horses and the extension of the work here so as to affect the 

 living tissue is a matter of comparatively slight divergence. So 

 near are these insects to their free congeners in habit and also in 

 structure that we may, with considerable assurance, consider that 

 the parasitic habit has been only recently assumed. 



In the Arachnoids the more constant parasitism of Derman ys- 

 sus and Argas is easily derivable from the less complete parasit 

 ism of Ixodes. Even in the strictly parasitic Sarcoptidas there is 

 every evidence to indicate their derivation from free forms. 



Prof. H. Garman, in a paper upon the origin and development 

 of parasitism among the Sarcoptidae, read at the meeting of the 

 American Association for the Advancement of Science held in 

 Washington in August, 1891 . has advanced a most interesting 

 theory to account for the origin of the parasitic habit in this fam 

 ily of mites. After discussing the recent theory that the mite are 

 descended from the true worms through such forms as Pentasto- 

 mum and Demodex, and deciding that the whole class of Arach- 

 nida owes its origin rather to the Crustacea, he compares the 

 structure of the parasitic family Sarcoptidae with the plant-feed 

 ing families and decides that they have probably originated from 

 the Tyroglyphidae, a large and injurious family, the species of which 

 feed exclusively upon vegetal matter. As an illustration of his 

 theory of the origin of the parasitic habit, he takes for example 

 species whose normal food is dead vegetal matter. In time of 

 scarcity of such food, animal refuse may bean enforced substitute, 

 or such food may occur constantly among the normal food of the 

 species, and certain individuals may gradually acquire a fondness 

 for it. Then, in a period of scarcity of vegetable food, those which 

 have accustomed themselves to animal food have the advantage, 



