Kilayim, Chapter 1, Mishnah 4. As in the previous mishnayos, here too the Mishnah lists pairs of items that are not kilayim with one another, except that now we move on to discuss the fruits of trees, as opposed to things that grow from the ground.
Fruit trees that are not kilayim with one another:
"Ha'agasim v'hakrustemelin" - the agasim are the fruit of the pear tree, and the krustemelin are a kind of small wild pear. They are not kilayim with one another.
"V'haparishin v'ha'uzradin" - the parishin are quinces, and the uzradin are a kind of small wild apple. They too are not kilayim with one another.
From here on the Mishnah takes the opposite approach from the one we have dealt with until now, and lists items which, even though they resemble one another and a person would have thought they are a single species, are in fact not a single species:
Fruit trees that are kilayim with one another:
"Hatapuach v'hachizrad" - the apple, and a particular kind of wild apple. Even though they resemble each other, they are kilayim with one another.
"Haparskim v'hashkedim" - the parskim are peaches, and the shkedim are almonds. One might have thought that they are a single species, but they are not.
"V'hashizfin v'harimin" - the shizfin are jujubes, and some understand that the reference is to a certain graft between olives and pomegranates; and the rimin - some understand that these are the lotus.
Regarding the pair of peaches and almonds, it is worth considering the question: what is the connection between peaches and almonds at all? Well, inside the pit of the peach one finds that same small almond, and indeed, botanically the two are considered the same genus - genuine cousins. Even so, the Mishnah informs us that almonds and peaches, despite the closeness between them, are considered two different species.
In summary: regarding all these pairs the Mishnah establishes "af al pi shedomin zeh lazeh - kilayim zeh bazeh" - even though they resemble one another, they are kilayim with one another: the external resemblance is not what determines the matter, and one may not mix one species with another. We have therefore learned two sides in this mishnah: pairs of fruit trees that are not kilayim with one another (agasim and krustemelin, parishin and uzradin), and pairs that resemble one another and are nevertheless kilayim (the apple and the chizrad, peaches and almonds, shizfin and rimin).