This is a photograph of chihuahuas representing Mexico, from Walter Esplin Mason's Dogs of All Nations (1915), as they appeared at the Panama-Pacific International Exposition in San Francisco.
The text asserts they weigh only one and a half to four pounds, so the desire to think of them as even smaller than they are has always been with us. It further states, "This dog is strictly of Mexican origin though it is also found in the state of Texas. It is the most diminutive of all breeds. Is remarkably game and exclusive in its affections. The breed is famous as a performing dog, being very intelligent and agile."
Apart from identifying the dogs as performers, the identity of the act is left out of the book. I can't imagine it is anyone else but Vaudeville singer Rosina Casselli who toured from England and Europe to the United States and Canada, and even Australia and New Zealand, as "Rosina Casselli and Her Midget Wonders."
A review of her show in New Zealand in 1909 described the dogs' abilities in detail: "Her troupe of midget chihuahua dogs, natives of Mexico, were beautiful animals which appeared to have the intelligence of human beings. They were driven on stage in a pretty motor-car by a smart little canine, which acted the chauffeur to perfection. Upon arrival, the troupe sprang to the ground, and the driver gave utterance to a joyous little bark, and steered his car off again.
"The midgets then to set to work. Their feats included seemingly impossible performances on horizontal bars, perpendicular rods, the trapeze, and other apparatus. Dogs sprang from bar to bar cleverly, just as a human gymnast might do, did wonders on the trapeze, hanging singly and in couples, turned somersaults, and exhibited innumerable showy tricks, some of them designed with a spice of humour which the quadrupeds seemed to appreciate.
"At the conclusion of the turn, the motor-car arrived, the chauffeur barked, and they sprang on board. A diverting altercation took place between a pair of beauties for one of the seats. Shortly afterwards, an explosion was heard, and presently, walking on its hind legs, came a solitary chihuahua. He was followed by the car, drawn by one of the other dogs, the accident causing a most amusing scene."