There is very good reason to believe that this migration has been going on from time immemorial, and consequently we find a few forms typical of the bygone times, left to struggle for existence with the more recent immigrants from shallow waters. But after all the proportion of ancient forms to modern ones in the fauna of the abyss is not larger than it is in the fauna of fresh-water lakes and streams or even of the dry land. Nor is there any reason why it should be. The land and the fresh water have been peopled by migrations from the shallow water of the sea from generation to generation in precisely the same way, and they each can show a certain number of archaic forms. We must now consider briefly some of the most interesting deep-sea representatives of the various classes of the animal kingdom, referring as we pass on to the extent to which these classes contribute to the fauna of the abyss. We find a great difficulty in determining with any degree of certainty the actual depths at which the supposed abysmal forms of Protozoa actually live. All the Radiolaria and Foraminifera—the only Protozoa that are largely represented in the fauna of the open seas—are planktonic in habit; that is to say, they float or drift about in the water without ever becoming attached to the sea bottom; and when the contents of a dredge, that has been hauled up from a great depth, are examined, it is impossible to say at what points in its long journey from the bottom the Protozoa it contains were caught. Even if dredges and nets are used which can be closed by a messenger at any particular depth, the problem cannot be very easily settled; for even if the protozoa shells that are captured are found to contain a certain amount of protoplasm, it must be proved that that protoplasm is actually alive when brought on deck before we know for certain that the species actually live on the bottom. When the pelagic Foraminifera and Radiolaria die and sink to the bottom, their protoplasm probably disintegrates very slowly, and it is quite probable that the floor of the ocean is littered with the shells of truly pelagic protozoa, each containing a greater or smaller amount of undecomposed protoplasm.