402 



THE AMERICAN NATURALIST [Vol. XL VIII 



from the former except by a narrow isthmus on the right 

 side. The long tail is usually without pigment, or mainly 

 so where areal restriction is present, and it is seldom that 

 pigment extends far on to the base of this member when 

 the restrictive tendency appears. In the domesticated 

 varieties of rats, the same patches may be distinguished. 

 There is, however, an interesting variety known as the 

 "hooded" rat, in which the ear and neck patches appear 

 to be normal, but a narrow median dorsal area is pig- 

 mented for a varying length, sometimes quite to the root 

 of the tail. A separate factor seems here to be involved, 

 producing what may be called a "centrifugal" type of 

 pigmentation, which in many forms of mammals causes a 

 black spine stripe (Sorer irardi, Tupaia tana, certain 

 forms of Apodemus, Equus cahallus), and others. 



Among guinea-pigs the typical primary patches are 

 beautifully shown and may be seen in sundry figures 

 published in papers by Professor Castle on heredity in 

 this animal. The guinea-pig is one of the few mammals 

 yet known in which the median crown patch is visibly 

 present, a character which I take to be primitive. 



In guinea-pigs the breaking up of the ticked color 

 pattern has progressed under long domestication to an 

 extraordinary degree, so that not only are black, tawny 

 or grizzled animals produced in various shades, but even 

 in the same individual, the different primary pigment 

 areas may be of different colors. This fact is of much 

 significance, for it indicates not only the mutual independ- 

 ence of the contiguous color areas, but further points to 

 the manner in which a variegated color pattern may have 

 been acquired. Among mammals the color pattern is in 

 general, not greatly developed in comparison with birds, 

 yet in many cases where some modification lias taken 

 place, it is evident that this differentiation is confined to 

 the limits of one or two of the primary pigment patches. 

 Thus in the South American Tayra (T, barbara), the 

 head and neck are a grizzled gray, and the breaks occur- 

 ring in pied individuals show that the grizzled condition 



