I drew a blank on the subject until I remembered that I'd always heard of them referred to as Rock Cornish Game Hens rather than just Cornish Game Hens. That's when I finally hit pay dirt.

The following is from a book by the brothers Lobel all about meat and poultry.

These delectable little birds were actually "invented". It took a printer of exceptional engravings++with a taste for fine art and fine food++to dream up the idea. Jacques Makowsky and his wife, Therese, came to America via Russia and France. He practiced his art in New York City for eight years and then retired to a farm in Pomfret County, Connecticut. The wild beauty of his surroundings++plus his idleness++ caused him to call his farm Idle Wild. He decided to become a poultry farmer, but only of gourmet birds. He first raised Guineas, but later he and his wife, after much experimenting, came up with a crossbreed of Cornish game cocks and Plymouth Rock hens. The result was a plump little bird with all-white meat.

The first Rock Cornish game hen made its debut in 1950. Now they sell in the millions and are shipped all over the country to gourmet restaurants and fine butcher shops. Aside from the succulence of its white meat, it has a distinct gamy flavor. This is because the birds are given a high protein diet that includes such native Connecticut produce as cranberries, acorns, and other nuts. It is sometimes compared in appearance and taste with the less available quail and squab.

The usual weight is 1 pound, which is ample for an individual serving. In fact, half a bird is adequate for home parties++if the rest of the menu is abundant.

From "Meat" by the Lobel Brothers, edited by Alice Richardson. Hawthorne Books Inc., New York, 1971.