and that Life
in general has been more heterogeneously manifested as time has
advanced. Let us cite, in illustration, the one case of the
_Vertebrata_. The earliest known vertebrate remains are those of Fishes;
and Fishes are the most homogeneous of the vertebrata. Later and more
heterogeneous are Reptiles. Later still, and more heterogeneous still,
are Birds and Mammals. If it be said that the Palaeozoic deposits, not
being estuary deposits, are not likely to contain the remains of
terrestrial vertebrata, which may nevertheless have existed at that era,
we reply that we are merely pointing to the leading facts, _such as they
are_. But to avoid any such criticism, let us take the mammalian
subdivision only. The earliest known remains of mammals are those of
small marsupials, which are the lowest of the mammalian type; while,
conversely, the highest of the mammalian type--Man--is the most recent.
The evidence that the vertebrate fauna, as a whole, has become more
heterogeneous, is considerably stronger. To the argument that the
vertebrate fauna of the Palaeozoic period, consisting, so far as we know,
entirely of Fishes, was less heterogeneous than the modern vertebrate
fauna, which includes Reptiles, Birds, and Mammals, of multitudinous
genera, it may be replied, as before, that estuary deposits of the
Palaeozoic period, could we find them, might contain other orders of
vertebrata. But no such reply can be made to the argument that whereas
the marine vertebrata of the Palaeozoic period consisted entirely of
cartilaginous fishes, the marine vertebrata of later periods include
numerous genera of osseous fishes; and that, therefore, the later marine
vertebrate faunas are more heterogeneous than the oldest known one. Nor,
again, can any such reply be made to the fact that there are far more
numerous orders and genera of mammalian remains in the tertiary
formations than in the secondary formations. Did we wish merely to make
out the best case, we might dwell upon the opinion of Dr. Carpenter, who
says that "the general facts of Palaeontology appear to sanction the
belief, that _the same plan_ may be traced out in what may be called
_the general life of the globe_, as in _the individual life_ of every
one of the forms of organized being which now people it." Or we might
quote, as decisive, the judgment of Professor Owen, who holds that the
earlier examples of each group of creatures severally departed less
widely from archetypa