476 THE AMERICAN NATURALIST [Vol. XLVIII 



one fore leg, as well as in the midline of the thorax, mark- 

 ing nearly the anteroposterior limits of the shoulder 

 patch. The neck patches are not separated in this figure 

 but have become so in Fig. 48, so that a continuous line 

 of white runs from chin to chest. In Fig. 47 the shoulder 

 and the side patches have both failed to join ventrally, 

 and thus a broad white line is formed down the center of 

 the belly from the conjoined neck patches to the rump 

 patches. If all these breaks were to be present in a single 

 animal, there would be a narrowed white area along the 

 entire ventral side of the body from chin to anus, extend- 

 ing on to the lower side of the fore legs. Practically 

 this condition exists in another species of the same 

 genus, Streator's weasel (Mustela streatori) of the 

 Pacific Coast, in which the throat, chest and belly are 

 white but the width and boundaries of the white area are 

 very variable in different individuals. It is therefore in 

 a stage beyond that which the minks have reached, yet it 

 has not attained the stage in which the white area is of 

 definite and rather constant bounds, as in certain other 

 weasels, for example Mustela noveboracensis, in which 

 the white, of the belly extends nearly or quite to the 

 lateral border of the body, but in different individuals 

 varies slightly, and .1/. cicof/uai/i/, in which the white area 

 of the belly constantly extends to the lateral boundary of 

 the venter from throat to anus. This is the condition 

 toward which the mink is tending. 



Another interesting case in which a pattern mark ap- 

 pears to be evolving through the fixation of a primary 

 break between pigment patches is that of the so-called 

 tayra of South America (Tayra barbara) a large Muste- 

 lid. The Central American race (biologice) of this animal 

 is wholly black, but the typical subspecies of Brazil and 

 northern South America is subject to a varying amount 

 of reduction in pigmentation. Curiously, this takes place 

 at the posterior end of the neck patches or at the anterior 

 part of the shoulder patches. Three of five specimens in 

 the Museum of Comparative Zoology are marked in this 



