Here again we encounter the necessity of admitting for such results long periods of time.

Within the range of history no well-marked instance of such development has been witnessed, and we speak with hesitation of doubtful instances of extinction.

Yet in geological times myriads of evolutions and extinctions have occurred.
Since thus, within the experience of man, no case of metamorphosis or development has been observed, some have been disposed to deny its possibility altogether, affirming that all the different species have come into existence by separate creative acts.

But surely it is less unphilosophical to suppose that each species has been evolved from a predecessor by a modification of its parts, than that it has suddenly started into existence out of nothing.

Nor is there much weight in the remark that no man has ever witnessed such a transformation taking place.