182 



THE ORIGIN AND DEVELOPMENT OF PARASITISM AMONG THE 



SARCOPTIDiE.* 



By H. Garman, Lexington, Ky. 



Assuming the evolution of species through the action of natural selec- 

 tion as established, we are at once met, in considering the origin and 

 development of the parasitic habit in Sarcoptidte, with the ques- 

 tions, what were the originals of the family and did it diverge from 

 the non -parasitic mites before or after the order Acarina became es- 

 tablished ? In other words, were the originals of the Sarcoptidie mites 

 or "were they something else? A more intimate knowledge of the forms 

 we are considering will doubtless give us better ground than Ave now 

 have for conclusions on the subject. In the light of present knowledge 

 it seems altogether probable that the immediate ancestors of the Sar- 

 coptidse were not only mites, but that they differed little, if at all, 

 from species now in existence. 



In deciding the position of an animal among its fellows we are prone to 

 be influenced by the idea of our own supremacy, and to rate it by its 

 complexity of structure or by its intelligence as shown by habit. Sys- 

 tematists sometimes lose sight of the fact that evolution does not al- 

 ways mean an increase (^f structural perfection, but often of the reverse 

 process, a simpUfication and reduction. The simplicity due to degener- 

 ation from one cause or another is liable to be mistaken for the simplic- 

 ity of lack of differentiation ; and parasitic and other animals whose 

 parts have been reduced from disuse have sometimes been placed at 

 the beginning of a series, when their proper place, it may be, is at the 

 end. 



The Acarina seem to me a group of these degraded animals, and 

 their place, if this is true, is not at the beginning of the line of Arach- 

 nidan descent. I subscribe to Gegenbaur's remark (Comparative An- 

 atomy, English translation, 230, 1878) : 



There seems to be no doubt that degeueratiou is present in these, and is indicated 

 by the parasitism which obtains in most of the families. 



Eeasons for considering the Acarina degraded Arachnids apply with 

 the same force to the Sarcoptidte among Acarina. The fact that they 

 are parasitic and at the same time diverge but little from the free-living 

 mites is good evidence that they are not the stem upon which the order 

 has developed. The further fact that they are parasitic only upon mam- 

 mals and birds speaks for their being a family of recent appearance. 

 The geological history speaks to the same purj)ose of the order. 



This view of the position of the Acarina among Arachnida, and of 

 the Sarcoptidae among Acarina, calls for some notice of several aberrant 

 animals which are commonly placed with the Acarina, and frequently 

 in or near the Sarcoptidse, because of a supposed relationship between 

 the Sarcoptidse and the Vermes. 



* Read before Section F, A. A. A. S., at the Washington meeting, August, 1891. 



