Lind's predictions had been right and wrong.
He had been right that all his wives would become pregnant but it was his side that dictated the when, not them. Further, there were other issues he had not expected.
Mortals could not survive in the 4th layer. They would literally explode if they were brought up to it.
As such, all beasts nurtured their children in the womb, egg, whatever until they were at least a Soul Realm. Body Refining was a given just by the nutrients the mother gave them so it was not so complicated until the final step.
Depending on the particular beast race, the breakthrough to the Soul Realm usually coincided or was swiftly followed by birth.
Demons seemed to have a natural ability to reach that level as well. It likely had to do with their ability to be so high in combat power or maybe demonic Qi was simply inclined to reach the Demon Soul Realm faster.
