Subject to these allowances, the fauna of each geological period undoubtedly is intermediate in character, between the preceding and succeeding faunas.

I need give only one instance, namely, the manner in which the fossils of the Devonian system, when this system was first discovered, were at once recognised by palaeontologists as intermediate in character between those of the overlying carboniferous and underlying Silurian systems.

But each fauna is not necessarily exactly intermediate, as unequal intervals of time have elapsed between consecutive formations.