THE AMERICAN NATURALIST [Vol. XLIV 



such as voles. ground-squirrels and rabbits, and their reduction 

 or increase in numbers, as the ease may be. powerfully affects 

 the welfare of such species of mammals and birds as prey upon 

 them. In the ease of rabbits, destruction by epidemics is peri- 

 odical, occurring- apparently about once in seven years, although 

 exact data are wanting as to the regularity or frequency of these 



that only a few individuals arc left to perpetuale tbc species. 

 They are also followed, there is some reason to believe, by an 

 increased birthrate by which the stock is rapidly replenished. 

 This periodic destruction is considered by Mr. Nelson in relation 

 to the apparent opportunity thus afforded for the origination, 

 through isolation, of many strongly characterized forms, but he 

 fails to find evidence that this is an evolutionary force of much 

 importance. The Lfpus atiirriratius group, occupying the vast 

 wooded area from Nova Scotia to western Alaska, which has been 

 subjected to numberless recurring periods of extreme abundance 

 and extreme scarcity, presents only a few, and not strongly, 

 differentiated subspecies, owing, it is believed, to the leveling 

 influence of similarity of climate. 



In illustration, however, of the effects of complete isolation 

 under similar climatic conditions, the black jack rabbit of the 

 island of Kspiritu Santo is cited. This small island lies off La 

 Paz Lay. Lower California, onlv four miles from the mainland. 



ition and climate. The adjacent mainland is 

 form of the Lt pus californiois group, the is- 



isentially the same in size and cranial char- 

 in color as to be commonly referred to as the 



bit. The intensification of color, making it 



onspicuous," has not been ''protective," nor 

 detriment, presumably from the fact that no 

 or large bird of prey shares its habitat. 



