rms than the _Didelphia,_
which last are intercalary between the _Ornithodelphia_ and the
_Monodelphia_. To what point of the Palaeozoic epoch, then, must we,
upon any rational estimate, relegate the origin of the _Monotremata_?

The investigation of the occurrence of the classes and of the orders
of the _Sauropsida_ in time points in exactly the same direction.
If, as there is great reason to believe, true Birds existed in the
Triassic epoch, the ornithoscelidous forms by which Reptiles passed
into Birds must have preceded them. In fact there is, even at present,
considerable ground for suspecting the existence of _Dinosauria_ in
the Permian formations; but, in that case, lizards must be of still
earlier date. And if the very small differences which are observable
between the _Crocodilia_ of the older Mesozoic formations and those of
the present day furnish any sort of approximation towards an estimate
of the average rate of change among the _Sauropsida_, it is almost
appalling to reflect how far back in Palaeozoic times we must go,
before we can hope to arrive at that common stock from which the
_Crocodilia, Lacertilia, Ornithoscelida_, and _Plesiosauria_, which
had attained so great a development in the Triassic epoch, must have
been derived.

The _Amphibia_ and _Pisces_ tell the same story. There is not a
single class of vertebrated animals which, when it first appears,
is represented by analogues of the lowest known members of the same
class. Therefore, if there is any truth in the doctrine of evolution,
every class must be vastly older than the first record of its
appearance upon the surface of the globe. But if considerations of
this kind compel us to place the origin of vertebrated animals at
a period sufficiently distant from the Upper Silurian, in which the
first Elasmobranchs and Ganoids occur, to allow of the evolution of
such fishes as these from a Vertebrate as simple as the _Amphioxus_,
I can only repeat that it is appalling to speculate upon the extent to
which that origin must have preceded the epoch of the first recorded
appearance of vertebrate life.

Such is the further commentary which I have to offer upon the
statement of the chief results of palaeontology which I formerly
ventured to lay before you.

But the growth of knowledge in the interval makes me conscious of
an omission of considerable moment in that statement, inasmuch as it
contains no reference to the bearings of palaeontology upon