+--------+------------------------+---------+---------+----------+ | |Grade of (F₂) offspring,| Totals. | Means. |Standard | |Grade | upper group parents. | | |deviation.| | of F₁ +--+--+--+--+--+--+--+---+----+----+----+----+----+-----+ |parents.|3½| 4|4¼|4½|4¾| 5|5¼| 5½|L[1]|U[2]|L[1]|U[2]|L[1]|U[2] | +--------+--+--+--+--+--+--+--+---+----+----+----+----+----+-----+ | 4⅛ | | | 1| 1| 1| | 1| 1| 3 | 5 | |4.85| | | | 4¼ | | | 1| 6| 1| 1| 1| | 1 | 10 | |4.62| | | | 4⅜ | | | 2| | 1| 1| | | 2 | 4 | |4.56| | | | 4½ | 1| 2| 4|10| 6| 8| 4| 6| 6 | 41 | |4.78| | | | 4⅝ | | | 1| 1| 1| 2| 3| | 2 | 8 | |4.91| | | | +--+--+--+--+--+--+--+---+----+----+----+----+----+-----+ |Totals | | | | | | | | | | | | | | | | or | | | | | | | | | | | | | | | |means | 1| 2| 9|18|10|12| 9| 7| 14 | 68 |+.75|4.77|1.03| .44 | +--------+--+--+--+--+--+--+--+---+----+----+----+----+----+-----+ [1] Lower group offspring, -1¾ to +2 inclusive. [2] Upper group offspring, +3½ to +5½ inclusive. 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. [Illustration: 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, _F_₁ 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. 8075-8078, four F₂ progeny (grandchildren) of ♂ 8000 and ♀ 6176. 8075 is black hooded, grade ¾; 8076 is gray, +4¼; 8077 is gray, +4; 8078 is black, +5½. [Illustration: PLATE 2] PLATE 3. ♂ 103, an evenly marked rat of grade +3. A female rat of this same grade was mated with ♂ 8000 (the wild gray male). Two litters of _F_₂ grandchildren are shown in 8062-8067, and 8070-8074. 8062 is black hooded, +2; 8064 is gray hooded, +2; 8065 is black, +5; 8066 is gray, +5¼; 8067 is gray, +4½ (notice white feet). 8070 is black hooded, +2; 8071 is gray hooded, +1¾; 8072 is gray, +5; 8073 is black, +5½; 8074 is gray, +5¾. [Illustration: PLATE 3]