same. This most important generalisation was rapidly verified
and extended to all parts of the world accessible to geologists;
and now it rests upon such an immense mass of observations as to
be one of the best established truths of natural science. To the
geologist the discovery was of infinite importance as it enabled
him to identify rocks of the same relative age, however their
continuity might be interrupted or their composition altered.
But to the biologist it had a still deeper meaning, for it
demonstrated that, throughout the prodigious duration of time
registered by the fossiliferous rocks, the living population of
the earth had undergone continual changes, not merely by the
extinction of a certain number of the species which had at first
existed, but by the continual generation of new species, and the
no less constant extinction of old ones.

Thus the broad outlines of palaeontology, in so far as it is the
common property of both the geologist and the biologist, were
marked out at the close of the last century. In tracing its
subsequent progress I must confine myself to the province of
biology, and, indeed, to the influence of palaeontology upon
zoological morphology. And I accept this limitation the more
willingly as the no less important topic of the bearing of
geology and of palaeontology upon distribution has been
luminously treated in the address of the President of the
Geographical Section.<3>

The succession of the species of animals and plants in time
being established, the first question which the zoologist or the
botanist had to ask himself was, What is the relation of these
successive species one to another? And it is a curious