es with the continuity of organic development, and
produces those puzzling discordances so generally met with in geological
formations of marine origin. While a case of the kind now described
affords evidence of the origin of species complete and conclusive,
though on a necessarily very limited scale, the very rarity of the
conditions which are essential to such completeness serves to explain
why it is that in most cases the direct evidence of evolution is not to
be obtained.

Another illustration of the filling up of gaps between existing groups
is afforded by Professor Huxley's researches on fossil crocodiles. The
gap between the existing crocodiles and the lizards is very wide, but as
we go back in geological time we meet with fossil forms which are to
some extent intermediate and form a connected series. The three living
genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene
formation, and allied forms of another genus, Holops, in the Chalk. From
the Chalk backward to the Lias another group of genera occurs, having
anatomical characteristics intermediate between the living crocodiles
and the most ancient forms. These, forming two genera Belodon and
Stagonolepis, are found in a still older formation, the Trias. They have
characters resembling some lizards, especially the remarkable Hatteria
of New Zealand, and have also some resemblances to the
Dinosaurians--reptiles which in some respects approach birds.
Considering how comparatively few are the remains of this group of
animals, the evidence which it affords of progressive development is
remarkably clear.[184]

Among the higher animals the rhinoceros, the horse, and the deer afford
good evidence of advance in organisation and of the filling up of the
gaps which separate the living forms from their nearest allies. The
earliest ancestral forms of the rhinoceroses occur in the Middle Eocene
of the United States, and were to some extent intermediate between the
rhinoceros and tapir families, having like the latter four toes to the
front feet, and three to those behind. These are followed in the Upper
Eocene by the genus Amynodon, in which the skull assumes more distinctly
the rhinocerotic type. Following this in the Lower Miocene we have the
Aceratherium, like the last in its feet, but still more decidedly a
rhinoceros in its general structure. From this there are two diverging
lines--one in the Old World, the other in the New. In the former, to
which th