nivorous and herbivorous forms. But the _Monotremata_ are lower forms
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 a