Correlated variation is the term applied to the common case of two organs so interdependent that when the one varies the other varies like wise. Multiple p:r•ts. such as the segments of an earthworm, vary just as different individuals do and follow the same laws, and where one of these repeated parts varies, they all vary in the same direction. Thus, if a peculiar spot appears in one segment of a leech, it is apt to appear in the whole series of segments; also. a variety of a species characterized by one peculiarity is apt to be characterized by a whole series of peculiar ities. The variability of the various elements of a series or multiple repeated organ differs in the different cases. For example, the spots on the thoracic shield of a potato beetle are sometimes constant in size and occurrence; in others, the reverse. In the ease of linear series, like the series of teeth on the jaw. it is often true that the terminal members of the series are the most variable. In general, the rule holds that specific characters are more variable than generic ones. But the relation between species and varying characters is even closer, as examination of ex treme types will show. There is thus such a relation between the species of a genus that each, winle maintaining its peculiar modes, varies espe cially in the direction of the others. A species
remains constant only in a constant environment. This is illustrated in every estuary, where the molInsks up toward the fresher water are smaller than those in the open sea. Insects are along the seacoast. Si) plants growing in cellars are long, and the leaves small and far apart, as compared with those living in the sun light. Mountain plants are smaller than those of the valley. A passage from one part of the country to another shows that animals gradually become dissimilar, and even the individuals of the same species undergo a change which may be apparently quite independent of environment. This is shown in the Galapagos Islands. whose elimatie conditions :Ire practically the same, yet on each island the lizards are peculiar, and only one kind of lizard occurs on an islaml.
ButtiomtAeitv. I):r•win, The Variation of Ani mals and Haws Under Domestication (London, 2d ed., 1575) ; Galton, Natural Inheritance (ib., 1889) ; Bateson, Materials for the Study of 1 ariation (ib., 1894) ; Pearson, The Grammar of Science (ib., 2d ed., 1900).