Age group 1 and 2 include juvenal and subadult animals. Animals of age groups 3 through 6 are considered adult. Individuals of age groups 3 through 5, including as they do the great majority of the adult population, were the only age classes used in measuring geographic variation.
Quimby's (1951:69) data indicate that some mice produce litters at the age of approximately 2 months, when four-fifths grown. Therefore, sexual maturity is not always synonymous with morphological maturity.
MEASUREMENTS OF EXTERNAL PARTS.--Data presented here on _Z. hudsonius_ are those recorded by Quimby (1951) on specimens from Anoka County, Minnesota, and those obtained by me from museum specimens from Menominee and Keweenaw counties, Michigan.
According to Quimby (1951:65-66) the mean length [= body length] for three newly born _Z. hudsonius_ is 24.8 mm (24.0-25.5); at the end of the fourth week of growth the mean length averaged 64.4 mm and at the 13th week 77.6 mm. Rapid growth occurs during the first four weeks, with the mean length increasing approximately 2.6 times the size at birth. After the fourth week of development, growth proceeds at a slower rate; the mean length at 13 weeks is only 3.1 times greater than the mean length at birth.
In specimens assigned to age groups 1 and 2 the length of the body averaged 70 and 74.8 mm, respectively. The individuals of both groups are less than 13 weeks old if we assume that growth proceeds at the same rate in Michigan as it does in Minnesota.
In the specimens from Michigan of age groups 3, 4, 5, and 6 the average length of the body is 80.9, 83.7, 89.0, and 83.6, respectively.
According to Quimby (_loc. cit._), the average length of the tail for three _Z. hudsonius_ at birth was 9.2 mm. (8.5-10.0). During the first four weeks of development the tail grew rapidly and reached an average length of 92.0 mm, which was 10 times the length at birth. By the end of 13 weeks of development the average length of the tail for these three individuals was 119.6 mm or 12 times the average length at birth. The most rapid growth was early in development: 80 per cent of the growth of the tail occurred during the first month, after which growth proceeded at a much slower rate.