31)2 



THE AMERICAN NATURALIST [Vol. XL VIII 



species. Centering nearly at the lower part of the back 

 are the pleural or side patches, each of which pigments 

 the area from the shoulder to the lumbar region and ante- 

 rior part of the hind leg of either side. Last of all, the 

 two sacral or rump patches, each of which on its respect- 

 ive side pigments the buttocks and tail. In most species 

 these two patches are so closely associated that they tend 

 to remain fused dorsomedially, so as to give the appear- 

 ance, when reduced, of a single median patch at the base 

 of the tail. Their frequent bilaterality, however, indi- 

 cates the dual origin of such median patches. Each of 

 the lateral patches in its complete development extends 

 from the mid-dorsal to the mid-ventral line or those of 

 opposite sides may overlap slightly. Reduction usually 

 first appears mid-ventrally. 



It is probable that the retinas should also be considered 

 as an additional pair of patches, since morphologically 

 the eye is of dermal origin, and there is sometimes seen 

 a tendency to the formation of a small circumorbital 

 patch, which appears to break from the ear patch when 

 tli is is largely reduced. 



Pocock (1907) has pointed out that in black-and-tan 

 dogs the tan appears about the muzzle, along the sides 

 and on the limbs, while the blacker portions are more 

 dorsal. It may be added that in tricolor hounds, in which 

 the several primary patches are reduced, these are often 

 tan color at their several peripheries and black centrally. 

 In both cases, the explanation is simply that pigment 

 formation is less intense the farther away from the pri- 

 mary centers. 



The reason of the division of the body surface into 

 these independent areas of pigmentation does not here 

 concern me. It is no doubt the result of "physiological 

 causes, and it is rather suggestive that the several patches 

 correspond externally to important nerve centers or 

 groups of nerves. Thus the eye pigment corresponds to 

 the optic nerve, the aural patch to the auditory nerve, so 

 that these two great external sense organs of the head 

 have each their corresponding pigment patch. The neck 



