The system of Linnæus, as expanded and improved by modern zoölogists, has proved itself to be admirably adapted to the study of the living world; but it is much more difficult to apply it to the fossils, for they introduce a third dimension, so to speak, for which the system was not designed. This third dimension is the successive modification in time of a genetically connected series. The cumulative effect of such modifications is so great that only very elastic definitions will include the earlier and later members of an unbroken series. In attempting to apply the Linnæan system to the successive _faunas_ (_i.e._ assemblages of animals) which have inhabited the earth, palæontologists have employed various devices. One such method is to classify each fauna without reference to those which precede and follow it, but this has the great drawback of obscuring and ignoring the relationships, to express which is the very object of classification. Another and more logical method is to treat species and genera as though they belonged to the present order of things, for these groups, particularly species, were relatively short-lived, when regarded from the standpoint of geological time, and either became so modified as to require recognition as new species and genera, or died out without leaving descendants. Groups of higher rank, families, orders, etc., are treated as _genetic series_ and include the principal line or stock and such side-branches as did not ramify too widely or depart too far from the main stem. Under the first arrangement, the horses, a long history of which has been deciphered, would be divided into several families; under the second, they are all included in a single family. One of the most interesting results of palæontological study is the discovery that in many families, such as the horses, rhinoceroses and camels, there are distinct series which independently passed through parallel courses of development, the series of each family keeping a remarkably even pace in the degree of progressive modification. Such a minor genetic series within a family is called a _phylum_, not a very happy selection, for the same term had been previously employed in a much wider sense, as equivalent to the subkingdom. In both uses of the term the underlying principle, that of genetic series, is the same; the difference is in the comprehensiveness of meaning. It must be admitted that no method, yet devised, of applying the Linnæan scheme to the fossils is altogether satisfactory, and indeed it is only the breaks and gaps in the palæontological record which makes possible any use of the scheme. Could we obtain approximately complete series of all the animals that have ever lived upon the earth, it would be necessary to invent some entirely new scheme of classification in order to express their mutual relationships.