6 F. B. SUMMER AND H. H. COLLINS. 



Likewise, the animal, when seized by this member, makes what 

 appear like vigorous and well-directed efforts to accomplish the 

 severance. In P. maniculatus, on the other hand, the skin is 

 far less easily detached, and the animal has scarcely ever been 

 observed to undergo either bodily gyrations or torsion of the 

 tail. 



To what extent this detachment of the skin is to be regarded 

 as an "adaptive mechanism," which has arisen because of its 

 utility, we regard it as futile to discuss at present. A single 

 observation may be cited, which, however, can not be regarded 

 as throwing much light on the matter. One specimen of P. 

 boylei rowleyi was caught by the tail in a spring mouse-trap, 

 the body being uninjured. Under these conditions, the animal 

 w T as unable to escape by slipping off the skin and died of shock or 

 exposure. On the other hand, it must be stated that a consid- 

 erable proportion of the living mice of this species which were 

 lifted by the tail underwent this mutilation, in spite of consider- 

 able care being taken to prevent it. What happens to the 

 exposed portion of the tail after the detachment of the skin, 

 has not been observed. It probably dies and falls off. 



SCRIPPS INSTITUTION FOR BIOLOGICAL RESEARCH, 

 August 8, 1917. 



