hed-whale was like, and at the same time seems to connect these
thoroughly marine mammals with land-animals. Though already entirely
adapted to an aquatic mode of life, the teeth, skull and backbone of
Protocetus display so many differences from those of the later
whales and so many approximations to those of primitive, carnivorous
land-mammals, as, in a large degree, to bridge over the gap between the
two groups. Thus one of the most puzzling of palaeontological questions
is in a fair way to receive a satisfactory answer. The origin of the
whalebone-whales and their relations to the toothed-whales cannot yet be
determined, since the necessary fossils have not been discovered.

Among the carnivorous mammals, phylogenetic series are not so clear and
distinct as among the hoofed animals, chiefly because the carnivores are
individually much less abundant, and well-preserved skeletons are among
the prizes of the collector. Nevertheless, much has already been learned
concerning the mutual relations of the carnivorous families, and several
phylogenetic series, notably that of the dogs, are quite complete. It
has been made extremely probable that the primitive dogs of the Eocene
represent the central stock, from which nearly or quite all the other
families branched off, though the origin and descent of the cats have
not yet been determined.

It should be clearly understood that the foregoing account of mammalian
descent is merely a selection of a few representative cases and might be
almost indefinitely extended. Nothing has been said, for example, of
the wonderful museum of ancient mammalian life which is entombed in the
rocks of South America, especially of Patagonia, and which opens a
world so entirely different from that of the northern continents,
yet exemplifying the same laws of "descent with modification." Very
beautiful phylogenetic series have already been established among these
most interesting and marvellously preserved fossils, but lack of space
forbids a consideration of them.

The origin of the mammalia, as a class, offers a problem of which
palaeontology can as yet present no definitive solution. Many
morphologists regard the early amphibia as the ancestral group from
which the mammals were derived, while most palaeontologists believe
that the mammals are descended from the reptiles. The most ancient known
mammals, those from the upper Triassic of Europe and North America, are
so extremely rare and so very i