Among the lowest and most ancient marine organisms are the Foraminifera, little masses of living jelly, apparently structureless, but which secrete beautiful shelly coverings, often perfectly symmetrical, as varied in form as those of the mollusca and far more complicated. These have been studied with great care by many eminent naturalists, and the late Dr. W.B. Carpenter in his great work--the _Introduction to the Study of the Foraminifera_--thus refers to their variability: "There is not a single species of plant or animal of which the range of variation has been studied by the collocation and comparison of so large a number of specimens as have passed under the review of Messrs. Williamson, Parker, Rupert Jones, and myself in our studies of the types of this group;" and he states as the result of this extensive comparison of specimens: "The range of variation is so great among the Foraminifera as to include not merely those differential characters which have been usually accounted _specific_, but also those upon which the greater part of the _genera_, of this group have been founded, and even in some instances those of its _orders_."[16] Coming now to a higher group--the Sea-Anemones--Mr. P.H. Gosse and other writers on these creatures often refer to variations in size, in the thickness and length of the tentacles, the form of the disc and of the mouth, and the character of surface of the column, while the colour varies enormously in a great number of the species. Similar variations occur in all the various groups of marine invertebrata, and in the great sub-kingdom of the mollusca they are especially numerous. Thus, Dr. S.P. Woodward states that many present a most perplexing amount of variation, resulting (as he supposes) from supply of food, variety of depth and of saltness of the water; but we know that many variations are quite independent of such causes, and we will now consider a few cases among the land-mollusca in which they have been more carefully studied. In the small forest region of Oahu, one of the Sandwich Islands, there have been found about 175 species of land-shells represented by 700 or 800 varieties; and we are told by the Rev. J.T. Gulick, who studied them carefully, that "we frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms; _and a full set of the varieties of each species presents a minute gradation of forms between the more divergent types found in the more widely separated localities_."