EXPLANATION OF PLATES.
Plate 1.
Top row, a set of arbitrary grades used in the classification of the rats studied.
Middle and bottom rows, skins of rats graded as indicated by the numerals above each skin. The animals graded, +4, +4¼ and +4¾, being entirely dark above, are shown in ventral view.
PLATE 1
Plate 2.
♂ 8000, a wild gray rat caught in Cambridge, Mass., October, 1908.
♀ 6176, a black hooded rat of grade -1¾ which was mated with ♂ 8000.
♂ 8021 and ♂ 8018, F₁ offspring of the pair just described, ♂ 8000 and ♀ 6176. ♂ 8021 is of grade +4¾, with considerable white below. Notice also his white legs and compare with those of his father. ♂ 8018 is of grade +5½. Notice white areas on belly and front legs.