Kilayim, Chapter One, Mishnah Four
This mishnah begins to deal with kilayim in trees, meaning which trees may not be grafted one onto the other. Note that this section does not deal with planting, because there is no prohibition of planting different trees in juxtaposition. The only prohibition is grafting.
The first section of the mishnah continues to list trees that are not kilayim one with the other. That is, they are nearly the same species. In the second section, the mishnah begins to list trees that are indeed kilayim with one another. Although they look similar and are in some ways similar, they are distinct enough that one cannot graft them together.
1) As for trees: the pear and the crustumenian pear, the medlar and azarolus, do not constitute kilayim one with the other.
2) The apple and the crab-apple, or the peach and almond, or the jujube and rimin, even though they are similar one to the other, they nevertheless constitute kilayim one with the other.