If the Sky fall we shall have Larks.

James Howell (c. 1594–1666) Welsh historian and writer

Paroimiographia [Παροιμιογραφία]: Proverbs, or, Old Sayed Sawes & Adages, “English Proverbs” (1659) [compiler]
    (Source)

See also Heywood (1546). The same page contains a variant:

When the Sky falls we shall catch Larks.