Testing for a relationship between the body size of animals and a gradient
such as latitude is complicated by the fact that typically there is a singl
e average size for each species, and each species occurs at several sample
stations over the gradient. This results in standard tests for statistical
significance being invalid. This problem can be overcome by using a randomi
zation test. A more difficult problem, however, is determining whether the
relationship between size and latitude is the same for two subfamilies of s
pecies. In this paper a general method for relating body size to latitude a
nd subfamily differences is proposed, with the significance of effects dete
rmined by randomization. A simulation study suggests that this procedure ha
s good properties. This approach to data analysis has promise both for the
particular situation considered and for other related problems in biogeogra
phy.