But it will make matters plainer, if we illustrate what is meant by the discontinuity or incompleteness of the fossil record, by reference to some concrete series, such as the so-called Pedigree of the Horse. Whenever a series of fossils, arranged in the order of their historical sequence, exhibits a gradation of increasing resemblance to the latest form, with which the series terminates, such a series is called a palæontological pedigree, and is said to represent so many stages in the racial development or phylogeny of the respective modern type. The classical example of this sort of “pedigree” is that of the Horse. It is, perhaps, one of the most complete among fossil “genealogies,” and yet, as has been frequently pointed out, it is, as it stands, extremely incomplete. Modern representatives of the _Equidae_, namely, the horse, the ass and the zebra, belong to a common genus, and are separated from one another by differences which are merely specific, but the differences which separate the various forms, that compose the “pedigree of the Horse,” are generic. We have, to borrow Gerard’s simile, nothing more than the piers of the evolutionary bridge, without the arches, and we do not know whether there ever were any arches. There is, indeed, a sort of progression, _e.g._, from the four-toed to a one-toed type, so that the morphological gradation does, in some degree, coincide with temporal succession. But, on the other hand, the fossil forms, interpreted as stages in the phylogeny of the Horse, are separated from one another by gaps so enormous, that, in the absence of intermediate species to bridge the intervals, it is practically impossible, particularly in the light of our experimental knowledge of Genetics, to conceive of any transition between them. Nor is this all. The difficulty is increased tenfold, when we attempt to relate the _Equidae_ to other mammalian groups. Fossil ungulates appear suddenly and contemporaneously in the Tertiary of North America, South America and Europe, without any transitional precursors, to connect them with the hypothetical proto-mammalian stock, and to substantiate their collaterality with other mammalian stocks.