change only such animals and plants as have a wide climatic range or can migrate freely. It is these species, and these only, which will be living on the neighbouring lands; it is only an assemblage like this that can stand the climatic alternations and relapses that are likely to attend the transition. An assemblage consisting only of species widely distributed in latitude is probably an assemblage that has special means of dispersal—even if we do not happen yet to have discovered these means.
These considerations should lead us to expect to find living, in any country which has lately undergone a change of climate, a somewhat peculiar assemblage, consisting mainly of hardy forms of wide range in latitude, and not characteristically either northern or southern. Mingled with them, we might expect a few survivors from the previous warm or cold period. A hardy fauna and flora seem to characterise the period of the submerged forests; but the absence or great scarcity of characteristic survivors from a former period suggests that even the lowest of these deposits is far removed from the Glacial Epoch. The arctic species had already had time to die out, or had been crowded out; but the time had not been sufficiently long for the incoming of the southern forms which now characterise our southern counties. Then, even less than now, had we reached a perfect adjustment of the fauna and flora to the climatic
