Terror of the Vervoids

MEL: How could they? It's obscene.

DOCTOR: Not to a Vervoid.

MEL: You can't justify it. It's just-

DOCTOR: It's a matter of perspective, Mel. In your house in Pease Pottage, you had a large garden. What did you do with the plants and weeds you uprooted?

MEL: Put them on a compost heap.

DOCTOR: They're obeying instinct, like migrating birds or salmon swimming relentlessly upstream to spawn even though they may die. A compulsive following of the life cycle. Come on, Mel.