That the domestic hog in some places in northern Luzon does thus interbreed with the wild ones is a proved fact.
"The Bontoc Igorot"
Albert Ernest Jenks
They also frequently interbreed.
"The History of the European Fauna"
R. F. Scharff
Next they may again, perhaps upon different roads, return to the old home where they find the old type likewise changed, both having become so different that they can interbreed no longer.
"Darwin and Modern Science"
A.C. Seward and Others