Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Necessary

As necessary as breathing is to live.
—Anonymous

Necessary as for the butterfly to escape from the worm to become a butterfly.
—Anonymous

As necessary as skin to the apple.
—Anonymous

As necessary as Churches.
—Robert Burton

Necessary … as friction in mechanics.
—C. C. Colton

Necessary as one’s digestion.
—Thomas Heywood

Necessary … as rakehells in an army.
—John Webster

Necessary … as the gargoyle on the spire and the high altar are necessary parts of a Gothic cathedral.
—H. G. Wells