Get action. Seize the moment. Man was never intended to become an oyster.

Theodore Roosevelt (1858-1919) American politician, statesman, conservationist, writer, US President (1901-1909)

Comment to Mrs. J. A. Roosevelt (25 Dec 1851)
    (Source)

Quoted in David McCullough, Mornings on Horseback (1981), sourced from the W. Sheffield Cowles, Jr. Collection (private). Usually given as a quote in full to his children, McCullough only notes the last sentence ("Man ... oyster") as an actual quotation.