Phylogeny of the Races

Before the Caliemhan, only the Primaeval races inhabited Geshkara: the primaeval elves and "skochets". Extremely little is known about the skochets except that they lived at polar latitudes.
Though there is still great debate over the exact nature of the Caliemhan and what it did, it is clear that after it the chilovak emerged and quickly drove off the primaeval elves. Following the Caliemhan, the primaeval elves split into three groups: one fled underground, seeking the protection of the dwarves, who had been uninvolved in the Caliemhan, and were enslaved by them, developing into the arpids: arps and gaunt-men, one fled into flying cities that eventually either fell or were abandoned, and the final fled west to "the Land of the Sunset", likely referring to the continent of Ulera, where elven populations still live today. It's likely, as has been observed in Ulera, that the chilovak outbred and outcompeted the elves, who are known for their long lives, but also long gestation times and sexual developments, which would have given a selective edge to the chilovak, who are assumed to have had lifespans similar to that of the modern Chilovakian races, the shortest-lived being the orcs and the longest-lived being the fire newts (or dynitians if considered Chilovakian).
Once the chilovak were established in the Old World, they developed, evolved, transmogrified, or otherwise radiated into the Chilovakian races: akridians, anthropoids (cnissites, horils, humans, and orcs), mustelins, saurians (fire newts, kobolds, and lizardfolk, as well as possibly the kaschehane), and dynitians, if they are considered a separate race. The exact position of what the fogmen were is unclear, as they appear to have been neither Chilovakian, nor the mysterious "skochets", and do not appear until well after the Caliemhan.
Of the Chilovakian races, only horils and humans would go on to develop advanced societies and culture, and while horils were the first to do so, humans have become by far the most succesful of the races since the Caliemhan.

Interbreeding

Arps/Guant-men

Arps and guant-men are fully capable of interbreeding and creating viable offspring, though such instances are exceedingly rare.

Humans/Horils

Humans and horils are fully capable of interbreeding and creating viable offspring. Such occurences were common in the past and are partly responsible for the small Horil population, as much of it dissolved into the human population. Indeed, most south Sossians likely have some amount of horil ancestry. The small population size and insular nature of modern horils means such crosses are rare today.

Horils/Orcs

Humans and horils are likely fully capable of interbreeding and creating viable offspring. As horils are native to the far south of Sossis and Orcs to the far north of Sossis and Eussis, such crosses unheard of in modern history. Indeed the only recorded instances of likely horil-orc crossings exist in Bronze Age horil texts.

Humans/Orcs

Humans and horils are fully capable of interbreeding and creating viable offspring. Historically the mutual hatred and distrust between humans and orcs has made such instances very rare. Indeed in modern times such relationships are still heavily stigmatised in both societies.

Anthropoids/Mustelins

The three extant anthropoid races (horils, humans, and orcs) are all likely capable of interbreeding with mustelins, though the offspring of such encounters are likely infertile. Crosses between humans and mustelins historically have been and in modern times still are relatively common, creating infertile offspring. As mustelins are native to Aresra and horils and orcs to Oecumene, such crosses did not happen before the Chaspethian Exchange, and none have been recorded in modern times.

Kobolds/Lizardfolk

Kobolds and lizardfolk are capable of interbreeding and creating fertile offspring, however, the significant size differences between the two races means that in the case of a lizardfolk father and kobold mother, the foetus will more than likely kill the mother. Such crossings were forbidden in the historic Blackrock Dracates. They are a relative rarity in central Aresra, but certainly do happen.

Non-viable

All other crossings do or did not produce offspring, or the possibility is unknown. It is possible, for example, that crosses between fire newts and the other saurian races were possible, but there is no record of such occurences.

Primaeval

• Primaeval Elves
• Arpids
• Arps
• Gaunt-men
• Elves
• Skochets†

Chilovakian

• Akridians
• Anthropo-Mustelids
• Anthropoids
• Cnissites†
• Horils
• Humans
• Orcs
• Mustelins
• Saurians
• Fire Newts†
• Kaschehane?†
• Kobolds
• Lizardfolk

Unclassified

• Dwarves†
• Dynitians
• Fogmen†
• Karbikti?†