Primula which are Self-Fertile
The genus Primula is famous in Botany for containing many species which are heterostylous - They produce seed primarily by crossing pollen between 'pin-flowered' plants and 'thrum-flowered' plants.
A 'pin' is a plant which holds the anthers deep down in the flower and the stigma high above them. A thrum does the opposite, and the anthers can be seen appearing at or near the mouth of the flower, while the stigma may be invisible far below them.
Primula plants have a variety of ways to inhibit pollen from germinating between pin and pin and between thrum and thrum. Self-fertilization is also made more diifficult. This arrangement ensures that seed is strong.
Most species and varieties will allow some self-fertilization, but this is often very limited. And there are several species which are known to be totally or almost totally self-sterile (See List 2 below). This is why for many species and garden varieties you will need plants of both kinds, both pins and thrums, if you want them to self-sow in your garden.
However, some species are homostylous - They hold the stigma and the anthers at the same level, usually near the mouth of the flower. These primulas are often self-fertile, so if you know this, you need purchase only one specimen in order to get seed. (Of course, you probably learned in elementary school that it is still best if plants can be cross-pollinated to get stronger seed. This is true also of the homostylous primulas.)
List 1: Some Primula species which are known to be self-fertile
verticillata, mollis, japonica, halleri, laurentiana, magellanica, scotica, scandinavica, stricta
Both homostylous and heterostylous, but still self-fertile: floribunda, chungensis, cockburniana
List 2: Some Primula species which are known to be totally or almost completely self-sterile
(less than a 2% chance of either a pin or a thrum setting seed by itself)
vialii, pulverulenta (0%), florindae, sikkimensis, farinosa, modesta (0%), flaccida, elatior, cortusoides, polyneura (0%), calderiana, sonchifolia
Note that P. pulverulenta will cross with P. japonica in the garden, but then your seed will not be P. pulverulenta, but a hybrid.
The above information was taken from John Richards' book Primula, 2nd ed: Timber Press, 2003. Pages 42-63 .