Various things are suspicious of April 1.  From English Illustrated, 1896.

"April."  From Judy, Or The London Serio-Comic Journal, 1883.

Here's the first of April from Judy, Or The London Serio-Comic Journal, 1877.

It's all in a day's work around here: "There were abstract equations to be worked out; difficult analyses to be made; mystical keys to be fitted to still more mystical complications; and the whole so blended and woven together, that the loss of a single link of the marvellous chain would destoy all hope of ever attaining the wished-for result."  From Leonard Kip's "The Secret of Apollonius Septrio," Hannibal's Man and Other Tales, 1878.

"April First, All Fools' Day."  From 1907.

Here are some best wishes from 1901.

Standing on eggs.  From Popular Mechanics, 1913.

We just discovered that the same year we published How to Be Your Own Cat, one Katerina Sidorova wrote How to Stop Being Human: Guidelines for Becoming a Squirrel.  (Select pages of the squirrel book are scanned here.  You can peek inside the cat book here.)  Something is in the air!

Here's a precursor to The Log Lady of Twin Peaks.  From Popular Mechanics, 1931

A statue holds a book with no title, the book never having been written about an event that left no discernible trace.  From My Father's Bonus March by Adam Langer, 2009.

From Jugend, 1911.

From Over the Plum Pudding by John Kendrick Bangs, 1901. 

From One Summer by Blanche Willis Howard, 1877.

"Lookin' as handsome as the law allows."  From Everybody's Magazine, 1908.

"I'd sooner have a mouthful of cold mutton sitting on a chair, than a whole Yorkshire pie in a coal-hole" is another thing you'll never hear a vegetarian say.  From English Illustrated, 1894.

The cartoonist labeled the earth and Atlas as if we wouldn't know what they were.  By Robert Carter for the N.Y. Evening Sun, reproduced in Cartoons magazine, 1914.

"Thought-reading [via psychometry] to find out who sent it."  From Fun magazine, 1885.

"The spirit of play conquers life's sorrow and flees from vain care."  From Hunter College's Wistarion yearbook, 1930.

"Black John" chastising the witches, from Letters on Demonology and Witchcraft by Walter Scott and illustrated by George Cruikshank, 1830.