Mutation.

The peaceful years, and then the stormy time
When the perturbed Earth moans, and Death himself
Seems ready to seize all his prey, "to smite
Once and to smite no more." Not yet the end,
And still the labour of the God goes on:
Time sows and reaps, and men are born and die;
Moons wax and wane, and all is changing still
As in the dream of some mysterious Power,
A dream of joy and woe, obscure as life —
That vagrant melody still lapsing down
The aeons to our doom!