I believe that when the 1876 Act was first enacted it wasn't appreciated that the legal definition of an animal was very restrictive namely only cattle, horses, asses, mules, goats, pigs, sheep and dogs. This is the definition still used in the Road Traffic Acts, i.e. if you collide with a dog you are supposed to try and help the animal, find the owner, recompense them if you were at fault etc. If you hit a fox in the same circumstances, morally you have as much need to see it's cared for, but legally I'm not so sure.
I'm also not sure whether the 1876 act was amended during its initial passage through Parliament or subsequently, but (ultimately) it did extend the definition of animal to include all vertebrates. This meant that in Biology lessons schoolchildren could cheerfully cut up worms and cockroaches but that frogs had to be rendered decerebrate by a licensed technician.